diff --git a/README.md b/README.md
index b7f3751..4d7ece9 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,8 @@ cac env rm <name>                       # 删除环境
 cac env set [name] proxy <url>          # 设置 / 修改代理
 cac env set [name] proxy --remove       # 移除代理
 cac env set [name] version <ver>        # 切换版本
+cac env set [name] timezone <IANA_TZ>   # 设置时区，例如 America/Los_Angeles
+cac env set [name] language <locale>    # 设置语言，例如 en_US.UTF-8 或 en-US
 cac <name>                              # 激活环境（快捷方式）
 cac ls                                  # = cac env ls
 ```
@@ -125,7 +127,7 @@ cac ls                                  # = cac env ls
 | `cac env create <name> [-p proxy] [-c ver] [--clone] [--telemetry mode] [--persona preset]` | 创建环境（自动激活，`--telemetry transparent/stealth/paranoid` 控制遥测，`--persona macos-vscode/...` 用于容器） |
 | `cac env ls` | 列出环境 |
 | `cac env rm <name>` | 删除环境 |
-| `cac env set [name] <key> <value>` | 修改环境（proxy / version / telemetry / persona） |
+| `cac env set [name] <key> <value>` | 修改环境（proxy / version / timezone / language / telemetry / persona） |
 | `cac env check [-d]` | 验证当前环境（`-d` 显示详情） |
 | `cac <name>` | 激活环境 |
 | **自管理** | |
@@ -287,6 +289,8 @@ cac env rm <name>                       # remove environment
 cac env set [name] proxy <url>          # set / change proxy
 cac env set [name] proxy --remove       # remove proxy
 cac env set [name] version <ver>        # change version
+cac env set [name] timezone <IANA_TZ>   # set timezone, e.g. America/Los_Angeles
+cac env set [name] language <locale>    # set LANG, e.g. en_US.UTF-8 or en-US
 cac <name>                              # activate (shortcut)
 cac ls                                  # = cac env ls
 ```
@@ -310,7 +314,7 @@ Each environment is fully isolated:
 | `cac env create <name> [-p proxy] [-c ver] [--clone] [--telemetry mode] [--persona preset]` | Create environment (auto-activates, `--telemetry transparent/stealth/paranoid` for telemetry control, `--persona macos-vscode/...` for containers) |
 | `cac env ls` | List environments |
 | `cac env rm <name>` | Remove environment |
-| `cac env set [name] <key> <value>` | Modify environment (proxy / version / telemetry / persona) |
+| `cac env set [name] <key> <value>` | Modify environment (proxy / version / timezone / language / telemetry / persona) |
 | `cac env check [-d]` | Verify current environment (`-d` for details) |
 | `cac <name>` | Activate environment |
 | **Self-management** | |
diff --git a/cac b/cac
index 2d312f0..49db916 100755
--- a/cac
+++ b/cac
@@ -120,6 +120,129 @@ _proxy_reachable() {
     (echo >/dev/tcp/"$host"/"$port") 2>/dev/null
 }
 
+_locale_from_country_code() {
+    case "$1" in
+        US) echo "en_US.UTF-8" ;;
+        GB) echo "en_GB.UTF-8" ;;
+        AU) echo "en_AU.UTF-8" ;;
+        CA) echo "en_CA.UTF-8" ;;
+        SG) echo "en_SG.UTF-8" ;;
+        HK) echo "zh_HK.UTF-8" ;;
+        TW) echo "zh_TW.UTF-8" ;;
+        JP) echo "ja_JP.UTF-8" ;;
+        KR) echo "ko_KR.UTF-8" ;;
+        DE) echo "de_DE.UTF-8" ;;
+        FR) echo "fr_FR.UTF-8" ;;
+        ES) echo "es_ES.UTF-8" ;;
+        IT) echo "it_IT.UTF-8" ;;
+        PT|BR) echo "pt_BR.UTF-8" ;;
+        RU) echo "ru_RU.UTF-8" ;;
+        NL) echo "nl_NL.UTF-8" ;;
+        IN) echo "en_IN.UTF-8" ;;
+        *)  echo "en_US.UTF-8" ;;
+    esac
+}
+
+# Query timezone and locale from the current proxy exit IP.
+# Output: <timezone>\t<locale>\t<country_code>
+_geo_detect_tz_lang() {
+    local proxy_url="$1" ip_info detected_tz country_code
+    [[ -n "$proxy_url" ]] || return 1
+    ip_info=$(curl -s --proxy "$proxy_url" --connect-timeout 8 "http://ip-api.com/json/?fields=timezone,countryCode" 2>/dev/null || true)
+    [[ -n "$ip_info" ]] || return 1
+
+    read -r detected_tz country_code < <(
+        echo "$ip_info" | python3 -c "import sys,json; d=json.load(sys.stdin); print(d.get('timezone',''), d.get('countryCode',''))" 2>/dev/null || echo ""
+    )
+    [[ -n "$detected_tz" ]] || return 1
+
+    printf '%s\t%s\t%s\n' "$detected_tz" "$(_locale_from_country_code "$country_code")" "$country_code"
+}
+
+# Validate an IANA timezone name.
+_validate_timezone() {
+    local value="$1"
+    python3 - "$value" << 'PY'
+import sys
+from zoneinfo import ZoneInfo
+
+value = sys.argv[1].strip()
+if not value:
+    raise SystemExit(1)
+
+try:
+    ZoneInfo(value)
+except Exception:
+    raise SystemExit(1)
+
+print(value)
+PY
+}
+
+# Accept a POSIX locale (en_US.UTF-8) or a simple BCP 47 tag (en-US),
+# then normalize to a UTF-8 locale for LANG.
+_normalize_language() {
+    local value="$1"
+    python3 - "$value" << 'PY'
+import re
+import sys
+
+value = sys.argv[1].strip()
+if not value:
+    raise SystemExit(1)
+
+aliases = {
+    "C": "C.UTF-8",
+    "POSIX": "C.UTF-8",
+    "C.UTF-8": "C.UTF-8",
+    "C.UTF8": "C.UTF-8",
+}
+default_regions = {
+    "ar": "SA",
+    "cs": "CZ",
+    "de": "DE",
+    "en": "US",
+    "es": "ES",
+    "fr": "FR",
+    "hi": "IN",
+    "id": "ID",
+    "it": "IT",
+    "ja": "JP",
+    "ko": "KR",
+    "ms": "MY",
+    "nl": "NL",
+    "pl": "PL",
+    "pt": "BR",
+    "ru": "RU",
+    "th": "TH",
+    "tr": "TR",
+    "uk": "UA",
+    "vi": "VN",
+    "zh": "CN",
+}
+
+if value in aliases:
+    print(aliases[value])
+    raise SystemExit(0)
+
+normalized = value.replace("-", "_")
+match = re.fullmatch(r"([A-Za-z]{2,3})(?:_([A-Za-z]{2}|\d{3}))?(?:\.(?:UTF-?8|utf-?8))?", normalized)
+if not match:
+    raise SystemExit(1)
+
+language = match.group(1).lower()
+region = match.group(2)
+if region:
+    region = region.upper()
+else:
+    region = default_regions.get(language)
+    if not region:
+        raise SystemExit(1)
+
+print(f"{language}_{region}.UTF-8")
+PY
+}
+
 # Auto-detect proxy protocol (when user didn't specify http/socks5/https)
 # Usage: _auto_detect_proxy "host:port:user:pass" → returns a working full URL
 _auto_detect_proxy() {
@@ -1728,38 +1851,15 @@ _env_cmd_create() {
         fi
     fi
 
-    # Geo-detect timezone (single request via proxy)
+    # Geo-detect timezone and locale (single request via proxy)
     local tz="America/New_York" lang="en_US.UTF-8"
     if [[ -n "$proxy_url" ]]; then
         printf "  $(_dim "Detecting timezone ...") "
-        local ip_info
-        ip_info=$(curl -s --proxy "$proxy_url" --connect-timeout 8 "http://ip-api.com/json/?fields=timezone,countryCode" 2>/dev/null || true)
-        if [[ -n "$ip_info" ]]; then
-            local detected_tz country_code
-            read -r detected_tz country_code < <(echo "$ip_info" | python3 -c "import sys,json; d=json.load(sys.stdin); print(d.get('timezone',''), d.get('countryCode',''))" 2>/dev/null || echo "")
+        local geo_info detected_tz detected_lang country_code
+        if geo_info=$(_geo_detect_tz_lang "$proxy_url"); then
+            IFS=$'\t' read -r detected_tz detected_lang country_code <<< "$geo_info"
             [[ -n "$detected_tz" ]] && tz="$detected_tz"
-            if [[ -n "$country_code" ]]; then
-                case "$country_code" in
-                    US) lang="en_US.UTF-8" ;;
-                    GB) lang="en_GB.UTF-8" ;;
-                    AU) lang="en_AU.UTF-8" ;;
-                    CA) lang="en_CA.UTF-8" ;;
-                    SG) lang="en_SG.UTF-8" ;;
-                    HK) lang="zh_HK.UTF-8" ;;
-                    TW) lang="zh_TW.UTF-8" ;;
-                    JP) lang="ja_JP.UTF-8" ;;
-                    KR) lang="ko_KR.UTF-8" ;;
-                    DE) lang="de_DE.UTF-8" ;;
-                    FR) lang="fr_FR.UTF-8" ;;
-                    ES) lang="es_ES.UTF-8" ;;
-                    IT) lang="it_IT.UTF-8" ;;
-                    PT|BR) lang="pt_BR.UTF-8" ;;
-                    RU) lang="ru_RU.UTF-8" ;;
-                    NL) lang="nl_NL.UTF-8" ;;
-                    IN) lang="en_IN.UTF-8" ;;
-                    *)  lang="en_US.UTF-8" ;;
-                esac
-            fi
+            [[ -n "$detected_lang" ]] && lang="$detected_lang"
             echo "$(_cyan "$tz") $(_dim "($country_code)")"
         else
             echo "$(_dim "default $tz")"
@@ -1977,7 +2077,7 @@ _env_cmd_set() {
     # Parse: cac env set [name] <key> <value|--remove>
     # If first arg is a known key, use current env; otherwise treat as env name
     local name="" key="" value="" remove=false
-    local known_keys="proxy version telemetry persona"
+    local known_keys="proxy version telemetry persona timezone tz language lang"
 
     if [[ $# -lt 1 ]] || [[ "${1:-}" == "-h" ]] || [[ "${1:-}" == "--help" ]] || [[ "${1:-}" == "help" ]]; then
         echo
@@ -1986,6 +2086,12 @@ _env_cmd_set() {
         echo "    $(_green "set") [name] proxy <url>                       Set proxy"
         echo "    $(_green "set") [name] proxy --remove                  Remove proxy"
         echo "    $(_green "set") [name] version <ver|latest>            Change Claude version"
+        echo "    $(_green "set") [name] timezone <IANA_TZ|auto>         Set timezone"
+        echo "    $(_green "set") [name] timezone --remove               Remove timezone override"
+        echo "                                                          Accepts IANA names like America/Los_Angeles"
+        echo "    $(_green "set") [name] language <locale|tag|auto>      Set LANG locale"
+        echo "    $(_green "set") [name] language --remove               Remove LANG override"
+        echo "                                                          Accepts en_US.UTF-8 or en-US, normalizes to UTF-8 locale"
         echo "    $(_green "set") [name] telemetry <stealth|paranoid|transparent>"
         echo "                                                          Telemetry blocking: stealth (1p_events only), paranoid (max), transparent (none)"
         echo "    $(_green "set") [name] persona <macos-vscode|macos-cursor|macos-iterm|linux-desktop|--remove>"
@@ -2007,7 +2113,7 @@ _env_cmd_set() {
     _require_env "$name"
     local env_dir="$ENVS_DIR/$name"
 
-    [[ $# -ge 1 ]] || _die "usage: cac env set [name] <proxy|version|bypass> <value|--remove>"
+    [[ $# -ge 1 ]] || _die "usage: cac env set [name] <proxy|version|timezone|language|telemetry|persona> <value|--remove>"
     key="$1"; shift
 
     # Parse value or --remove
@@ -2047,6 +2153,58 @@ _env_cmd_set() {
             echo "$ver" > "$env_dir/version"
             echo "$(_green_bold "Set") version for $(_bold "$name") → $(_cyan "$ver")"
             ;;
+        timezone|tz)
+            if [[ "$remove" == "true" ]]; then
+                rm -f "$env_dir/tz"
+                echo "$(_green_bold "Removed") timezone override from $(_bold "$name")"
+            else
+                [[ -n "$value" ]] || _die "usage: cac env set [name] timezone <IANA_TZ|auto>"
+                if [[ "$value" == "auto" ]]; then
+                    local proxy_url geo_info detected_tz detected_lang country_code
+                    proxy_url=$(_read "$env_dir/proxy" "")
+                    [[ -n "$proxy_url" ]] || _die "cannot auto-detect timezone without a proxy configured"
+                    printf "  $(_dim "Detecting timezone ...") "
+                    if geo_info=$(_geo_detect_tz_lang "$proxy_url"); then
+                        IFS=$'\t' read -r detected_tz detected_lang country_code <<< "$geo_info"
+                        value="$detected_tz"
+                        echo "$(_cyan "$value") $(_dim "($country_code)")"
+                    else
+                        echo "$(_red "failed")"
+                        _die "failed to detect timezone from proxy exit IP"
+                    fi
+                else
+                    value=$(_validate_timezone "$value") || _die "invalid timezone '$value' (use an IANA name like America/Los_Angeles)"
+                fi
+                echo "$value" > "$env_dir/tz"
+                echo "$(_green_bold "Set") timezone for $(_bold "$name") → $(_cyan "$value")"
+            fi
+            ;;
+        language|lang)
+            if [[ "$remove" == "true" ]]; then
+                rm -f "$env_dir/lang"
+                echo "$(_green_bold "Removed") language override from $(_bold "$name")"
+            else
+                [[ -n "$value" ]] || _die "usage: cac env set [name] language <locale|tag|auto>"
+                if [[ "$value" == "auto" ]]; then
+                    local proxy_url geo_info detected_tz detected_lang country_code
+                    proxy_url=$(_read "$env_dir/proxy" "")
+                    [[ -n "$proxy_url" ]] || _die "cannot auto-detect language without a proxy configured"
+                    printf "  $(_dim "Detecting language ...") "
+                    if geo_info=$(_geo_detect_tz_lang "$proxy_url"); then
+                        IFS=$'\t' read -r detected_tz detected_lang country_code <<< "$geo_info"
+                        value="$detected_lang"
+                        echo "$(_cyan "$value") $(_dim "($country_code)")"
+                    else
+                        echo "$(_red "failed")"
+                        _die "failed to detect language from proxy exit IP"
+                    fi
+                else
+                    value=$(_normalize_language "$value") || _die "invalid language '$value' (use a locale like en_US.UTF-8, a tag like en-US, or 'auto')"
+                fi
+                echo "$value" > "$env_dir/lang"
+                echo "$(_green_bold "Set") language for $(_bold "$name") → $(_cyan "$value")"
+            fi
+            ;;
         telemetry)
             [[ "$remove" != "true" ]] || _die "cannot remove telemetry mode"
             [[ -n "$value" ]] || _die "usage: cac env set [name] telemetry <stealth|paranoid|transparent>"
@@ -2072,7 +2230,7 @@ _env_cmd_set() {
             fi
             ;;
         *)
-            _die "unknown key '$key' — use proxy, version, telemetry, or persona"
+            _die "unknown key '$key' — use proxy, version, timezone, language, telemetry, or persona"
             ;;
     esac
 }
@@ -2101,7 +2259,7 @@ cmd_env() {
             echo "    $(_green "create") <name> [-p proxy] [-c ver] [--telemetry mode] [--persona preset]"
             echo "                             Create isolated environment (auto-activates)"
             echo "    $(_green "set") [name] <key> <value>        Modify environment"
-            echo "                             proxy, version, telemetry, or persona"
+            echo "                             proxy, version, timezone, language, telemetry, or persona"
             echo "    $(_green "ls")              List all environments"
             echo "    $(_green "rm") <name>       Remove an environment"
             echo "    $(_green "check")           Verify current environment"
@@ -3500,6 +3658,7 @@ cmd_help() {
     echo "  $(_bold "Environment")"
     echo "    $(_green "cac env create") <name> [-p proxy] [-c ver]"
     echo "    $(_green "cac env set") [name] <key> <value>   Modify environment"
+    echo "                                     proxy|version|timezone|language|telemetry|persona"
     echo "    $(_green "cac env ls")                  List all environments"
     echo "    $(_green "cac env rm") <name>           Remove an environment"
     echo "    $(_green "cac env check")               Verify current environment"
diff --git a/docs/changelog.mdx b/docs/changelog.mdx
index ea1d8f7..95b2e2b 100644
--- a/docs/changelog.mdx
+++ b/docs/changelog.mdx
@@ -5,6 +5,17 @@ description: Release history and notable changes
 
 All notable changes to this project are documented here. Each entry links to the corresponding PR or issue.
 
+## Unreleased
+
+**New: `cac env set timezone|language` with validation and auto-detect**
+
+- Added `cac env set <name> timezone <IANA_TZ|auto>` and alias `tz`.
+- Added `cac env set <name> language <locale|tag|auto>` and alias `lang`.
+- Timezones are validated against Python `zoneinfo`, so invalid names are rejected early.
+- Languages accept either POSIX locales like `en_US.UTF-8` or simple BCP 47 tags like `en-US`, then normalize to UTF-8 locales for `LANG`.
+- `auto` re-detects timezone and language from the current proxy exit IP geolocation, making it easy to resync after changing proxies.
+- Help output, README, and command docs were updated to document the new behavior and examples.
+
 ## v1.5.4
 <sub>2026-04-02</sub>
 
diff --git a/docs/commands/env.mdx b/docs/commands/env.mdx
index 54ac4b1..390c6e2 100644
--- a/docs/commands/env.mdx
+++ b/docs/commands/env.mdx
@@ -119,6 +119,10 @@ If `name` is omitted, the currently active environment is used.
 
 **Shorthand:** `cac env <name> <key> <value>` (equivalent to `cac env set <name> <key> <value>`).
 
+<Info>
+`timezone` and `language` are validated before being written. `auto` re-detects from the current proxy exit IP. This is especially useful after changing proxies, because proxy changes do not silently rewrite existing `tz` or `lang` files.
+</Info>
+
 ### Proxy
 
 ```bash
@@ -139,6 +143,34 @@ cac env set work version 2.1.81    # pin a specific version
 cac env set work version latest    # resolve and pin the current latest
 ```
 
+### Timezone
+
+Use a standard IANA timezone name. `auto` re-detects from the current proxy exit IP.
+
+```bash
+cac env set work timezone America/Los_Angeles
+cac env set work timezone Asia/Shanghai
+cac env set work timezone auto
+cac env set work timezone --remove
+```
+
+### Language
+
+Use a UTF-8 locale for `LANG`. The command also accepts simple BCP 47 tags and normalizes them to a POSIX locale.
+
+```bash
+cac env set work language en_US.UTF-8
+cac env set work language en-US       # normalized to en_US.UTF-8
+cac env set work language zh-CN       # normalized to zh_CN.UTF-8
+cac env set work language auto
+cac env set work language --remove
+```
+
+Validation and normalization:
+- `timezone` must be a valid IANA timezone such as `America/Los_Angeles` or `Asia/Shanghai`
+- `language` accepts `en_US.UTF-8`, `zh_CN.UTF-8`, `en-US`, `zh-CN`, and similar forms
+- Language values are normalized to `ll_CC.UTF-8` before writing to the environment
+
 ### Telemetry mode
 
 Change telemetry blocking strategy after environment creation.
@@ -175,7 +207,7 @@ cac env check -d       # detailed output
 | `-d, --details` | Show detailed diagnostic output instead of just the summary. |
 
 Checks:
-- Environment info (proxy, UUID, version, timezone)
+- Environment info (proxy, UUID, version, timezone, language)
 - Proxy connectivity and exit IP (if proxy configured)
 - TUN conflict detection (if proxy configured)
 - Security protections (DNS guard, telemetry env vars, mTLS)
diff --git a/docs/zh/changelog.mdx b/docs/zh/changelog.mdx
index dc500a2..7f89aa9 100644
--- a/docs/zh/changelog.mdx
+++ b/docs/zh/changelog.mdx
@@ -5,6 +5,17 @@ description: 版本发布历史和重要变更
 
 所有重要变更记录在此，每个条目关联对应的 PR 或 Issue。
 
+## Unreleased
+
+**新增：`cac env set timezone|language`，带校验和自动探测**
+
+- 新增 `cac env set <name> timezone <IANA_TZ|auto>`，别名 `tz`。
+- 新增 `cac env set <name> language <locale|tag|auto>`，别名 `lang`。
+- 时区使用 Python `zoneinfo` 做 IANA 名称校验，非法值会直接拒绝。
+- 语言既支持 `en_US.UTF-8` 这种 POSIX locale，也支持 `en-US` 这种常见标签，并统一规范化为 UTF-8 locale 写入 `LANG`。
+- `auto` 会根据当前代理出口 IP 的地理信息重新探测时区和语言，方便在换代理后重新同步。
+- 帮助输出、README 和命令文档已同步更新。
+
 ## v1.5.4
 <sub>2026-04-02</sub>
 
diff --git a/docs/zh/commands/env.mdx b/docs/zh/commands/env.mdx
index a51be95..f1f9097 100644
--- a/docs/zh/commands/env.mdx
+++ b/docs/zh/commands/env.mdx
@@ -118,11 +118,19 @@ cac env set [name] <key> <value>
 
 不指定 `name` 时，默认修改当前活跃环境。
 
+<Info>
+`timezone` 和 `language` 会在写入前做校验。`auto` 会按当前代理出口 IP 重新探测。这个设计是有意的：修改代理时不会悄悄覆盖已有的 `tz` / `lang`，需要你显式同步，避免误改。
+</Info>
+
 | 用法 | 说明 |
 |:---|:---|
 | `cac env set [name] proxy <url>` | 设置或更换代理 |
 | `cac env set [name] proxy --remove` | 移除代理 |
 | `cac env set [name] version <ver\|latest>` | 更换 Claude Code 版本，`latest` 自动解析 |
+| `cac env set [name] timezone <IANA_TZ\|auto>` | 设置时区，或按当前代理出口自动探测 |
+| `cac env set [name] timezone --remove` | 移除时区覆盖，回退为宿主默认行为 |
+| `cac env set [name] language <locale\|tag\|auto>` | 设置语言，接受 `en_US.UTF-8` 或 `en-US` |
+| `cac env set [name] language --remove` | 移除语言覆盖，回退为宿主默认行为 |
 | `cac env set [name] telemetry <mode>` | 修改遥测屏蔽模式：`transparent` / `stealth` / `paranoid` |
 | `cac env set [name] persona <preset\|--remove>` | 修改或移除终端人设 |
 
@@ -141,6 +149,19 @@ cac env set work proxy --remove
 cac env set work version 2.1.81
 cac env set work version latest
 
+# 时区
+cac env set work timezone America/Los_Angeles
+cac env set work timezone Asia/Shanghai
+cac env set work timezone auto
+cac env set work timezone --remove
+
+# 语言
+cac env set work language en_US.UTF-8
+cac env set work language en-US
+cac env set work language zh-CN
+cac env set work language auto
+cac env set work language --remove
+
 # 遥测模式
 cac env set work telemetry stealth    # 屏蔽 1p_events，Feature flags 正常
 cac env set work telemetry paranoid   # 最大屏蔽
@@ -155,6 +176,11 @@ cac env work proxy 1.2.3.4:1080:user:pass
 cac env proxy --remove        # 移除当前环境代理
 ```
 
+校验与规范化规则：
+- `timezone` 必须是合法的 IANA 时区名，例如 `America/Los_Angeles`、`Asia/Shanghai`
+- `language` 接受 `en_US.UTF-8`、`zh_CN.UTF-8`，也接受 `en-US`、`zh-CN` 这类常见标签
+- `language` 写入环境前会统一规范化成 `ll_CC.UTF-8`
+
 ## check
 
 对当前活跃环境运行诊断检查。输出简洁的通过/失败结论摘要。
@@ -169,7 +195,7 @@ cac env check -d       # 详细输出
 | `-d, --details` | 显示详细诊断输出，而非仅摘要。 |
 
 检查项目：
-- 环境信息（代理、UUID、版本、时区）
+- 环境信息（代理、UUID、版本、时区、语言）
 - 代理连通性和出口 IP（如果配置了代理）
 - TUN 冲突检测（如果配置了代理）
 - 安全防护（DNS 守卫、遥测环境变量、mTLS）
diff --git a/src/cmd_env.sh b/src/cmd_env.sh
index 36993c1..d130a30 100644
--- a/src/cmd_env.sh
+++ b/src/cmd_env.sh
@@ -54,38 +54,15 @@ _env_cmd_create() {
         fi
     fi
 
-    # Geo-detect timezone (single request via proxy)
+    # Geo-detect timezone and locale (single request via proxy)
     local tz="America/New_York" lang="en_US.UTF-8"
     if [[ -n "$proxy_url" ]]; then
         printf "  $(_dim "Detecting timezone ...") "
-        local ip_info
-        ip_info=$(curl -s --proxy "$proxy_url" --connect-timeout 8 "http://ip-api.com/json/?fields=timezone,countryCode" 2>/dev/null || true)
-        if [[ -n "$ip_info" ]]; then
-            local detected_tz country_code
-            read -r detected_tz country_code < <(echo "$ip_info" | python3 -c "import sys,json; d=json.load(sys.stdin); print(d.get('timezone',''), d.get('countryCode',''))" 2>/dev/null || echo "")
+        local geo_info detected_tz detected_lang country_code
+        if geo_info=$(_geo_detect_tz_lang "$proxy_url"); then
+            IFS=$'\t' read -r detected_tz detected_lang country_code <<< "$geo_info"
             [[ -n "$detected_tz" ]] && tz="$detected_tz"
-            if [[ -n "$country_code" ]]; then
-                case "$country_code" in
-                    US) lang="en_US.UTF-8" ;;
-                    GB) lang="en_GB.UTF-8" ;;
-                    AU) lang="en_AU.UTF-8" ;;
-                    CA) lang="en_CA.UTF-8" ;;
-                    SG) lang="en_SG.UTF-8" ;;
-                    HK) lang="zh_HK.UTF-8" ;;
-                    TW) lang="zh_TW.UTF-8" ;;
-                    JP) lang="ja_JP.UTF-8" ;;
-                    KR) lang="ko_KR.UTF-8" ;;
-                    DE) lang="de_DE.UTF-8" ;;
-                    FR) lang="fr_FR.UTF-8" ;;
-                    ES) lang="es_ES.UTF-8" ;;
-                    IT) lang="it_IT.UTF-8" ;;
-                    PT|BR) lang="pt_BR.UTF-8" ;;
-                    RU) lang="ru_RU.UTF-8" ;;
-                    NL) lang="nl_NL.UTF-8" ;;
-                    IN) lang="en_IN.UTF-8" ;;
-                    *)  lang="en_US.UTF-8" ;;
-                esac
-            fi
+            [[ -n "$detected_lang" ]] && lang="$detected_lang"
             echo "$(_cyan "$tz") $(_dim "($country_code)")"
         else
             echo "$(_dim "default $tz")"
@@ -303,7 +280,7 @@ _env_cmd_set() {
     # Parse: cac env set [name] <key> <value|--remove>
     # If first arg is a known key, use current env; otherwise treat as env name
     local name="" key="" value="" remove=false
-    local known_keys="proxy version telemetry persona"
+    local known_keys="proxy version telemetry persona timezone tz language lang"
 
     if [[ $# -lt 1 ]] || [[ "${1:-}" == "-h" ]] || [[ "${1:-}" == "--help" ]] || [[ "${1:-}" == "help" ]]; then
         echo
@@ -312,6 +289,12 @@ _env_cmd_set() {
         echo "    $(_green "set") [name] proxy <url>                       Set proxy"
         echo "    $(_green "set") [name] proxy --remove                  Remove proxy"
         echo "    $(_green "set") [name] version <ver|latest>            Change Claude version"
+        echo "    $(_green "set") [name] timezone <IANA_TZ|auto>         Set timezone"
+        echo "    $(_green "set") [name] timezone --remove               Remove timezone override"
+        echo "                                                          Accepts IANA names like America/Los_Angeles"
+        echo "    $(_green "set") [name] language <locale|tag|auto>      Set LANG locale"
+        echo "    $(_green "set") [name] language --remove               Remove LANG override"
+        echo "                                                          Accepts en_US.UTF-8 or en-US, normalizes to UTF-8 locale"
         echo "    $(_green "set") [name] telemetry <stealth|paranoid|transparent>"
         echo "                                                          Telemetry blocking: stealth (1p_events only), paranoid (max), transparent (none)"
         echo "    $(_green "set") [name] persona <macos-vscode|macos-cursor|macos-iterm|linux-desktop|--remove>"
@@ -333,7 +316,7 @@ _env_cmd_set() {
     _require_env "$name"
     local env_dir="$ENVS_DIR/$name"
 
-    [[ $# -ge 1 ]] || _die "usage: cac env set [name] <proxy|version|bypass> <value|--remove>"
+    [[ $# -ge 1 ]] || _die "usage: cac env set [name] <proxy|version|timezone|language|telemetry|persona> <value|--remove>"
     key="$1"; shift
 
     # Parse value or --remove
@@ -373,6 +356,58 @@ _env_cmd_set() {
             echo "$ver" > "$env_dir/version"
             echo "$(_green_bold "Set") version for $(_bold "$name") → $(_cyan "$ver")"
             ;;
+        timezone|tz)
+            if [[ "$remove" == "true" ]]; then
+                rm -f "$env_dir/tz"
+                echo "$(_green_bold "Removed") timezone override from $(_bold "$name")"
+            else
+                [[ -n "$value" ]] || _die "usage: cac env set [name] timezone <IANA_TZ|auto>"
+                if [[ "$value" == "auto" ]]; then
+                    local proxy_url geo_info detected_tz detected_lang country_code
+                    proxy_url=$(_read "$env_dir/proxy" "")
+                    [[ -n "$proxy_url" ]] || _die "cannot auto-detect timezone without a proxy configured"
+                    printf "  $(_dim "Detecting timezone ...") "
+                    if geo_info=$(_geo_detect_tz_lang "$proxy_url"); then
+                        IFS=$'\t' read -r detected_tz detected_lang country_code <<< "$geo_info"
+                        value="$detected_tz"
+                        echo "$(_cyan "$value") $(_dim "($country_code)")"
+                    else
+                        echo "$(_red "failed")"
+                        _die "failed to detect timezone from proxy exit IP"
+                    fi
+                else
+                    value=$(_validate_timezone "$value") || _die "invalid timezone '$value' (use an IANA name like America/Los_Angeles)"
+                fi
+                echo "$value" > "$env_dir/tz"
+                echo "$(_green_bold "Set") timezone for $(_bold "$name") → $(_cyan "$value")"
+            fi
+            ;;
+        language|lang)
+            if [[ "$remove" == "true" ]]; then
+                rm -f "$env_dir/lang"
+                echo "$(_green_bold "Removed") language override from $(_bold "$name")"
+            else
+                [[ -n "$value" ]] || _die "usage: cac env set [name] language <locale|tag|auto>"
+                if [[ "$value" == "auto" ]]; then
+                    local proxy_url geo_info detected_tz detected_lang country_code
+                    proxy_url=$(_read "$env_dir/proxy" "")
+                    [[ -n "$proxy_url" ]] || _die "cannot auto-detect language without a proxy configured"
+                    printf "  $(_dim "Detecting language ...") "
+                    if geo_info=$(_geo_detect_tz_lang "$proxy_url"); then
+                        IFS=$'\t' read -r detected_tz detected_lang country_code <<< "$geo_info"
+                        value="$detected_lang"
+                        echo "$(_cyan "$value") $(_dim "($country_code)")"
+                    else
+                        echo "$(_red "failed")"
+                        _die "failed to detect language from proxy exit IP"
+                    fi
+                else
+                    value=$(_normalize_language "$value") || _die "invalid language '$value' (use a locale like en_US.UTF-8, a tag like en-US, or 'auto')"
+                fi
+                echo "$value" > "$env_dir/lang"
+                echo "$(_green_bold "Set") language for $(_bold "$name") → $(_cyan "$value")"
+            fi
+            ;;
         telemetry)
             [[ "$remove" != "true" ]] || _die "cannot remove telemetry mode"
             [[ -n "$value" ]] || _die "usage: cac env set [name] telemetry <stealth|paranoid|transparent>"
@@ -398,7 +433,7 @@ _env_cmd_set() {
             fi
             ;;
         *)
-            _die "unknown key '$key' — use proxy, version, telemetry, or persona"
+            _die "unknown key '$key' — use proxy, version, timezone, language, telemetry, or persona"
             ;;
     esac
 }
@@ -427,7 +462,7 @@ cmd_env() {
             echo "    $(_green "create") <name> [-p proxy] [-c ver] [--telemetry mode] [--persona preset]"
             echo "                             Create isolated environment (auto-activates)"
             echo "    $(_green "set") [name] <key> <value>        Modify environment"
-            echo "                             proxy, version, telemetry, or persona"
+            echo "                             proxy, version, timezone, language, telemetry, or persona"
             echo "    $(_green "ls")              List all environments"
             echo "    $(_green "rm") <name>       Remove an environment"
             echo "    $(_green "check")           Verify current environment"
diff --git a/src/cmd_help.sh b/src/cmd_help.sh
index 106b54c..8161aba 100644
--- a/src/cmd_help.sh
+++ b/src/cmd_help.sh
@@ -8,6 +8,7 @@ cmd_help() {
     echo "  $(_bold "Environment")"
     echo "    $(_green "cac env create") <name> [-p proxy] [-c ver]"
     echo "    $(_green "cac env set") [name] <key> <value>   Modify environment"
+    echo "                                     proxy|version|timezone|language|telemetry|persona"
     echo "    $(_green "cac env ls")                  List all environments"
     echo "    $(_green "cac env rm") <name>           Remove an environment"
     echo "    $(_green "cac env check")               Verify current environment"
diff --git a/src/utils.sh b/src/utils.sh
index d89141a..5d08f95 100644
--- a/src/utils.sh
+++ b/src/utils.sh
@@ -110,6 +110,129 @@ _proxy_reachable() {
     (echo >/dev/tcp/"$host"/"$port") 2>/dev/null
 }
 
+_locale_from_country_code() {
+    case "$1" in
+        US) echo "en_US.UTF-8" ;;
+        GB) echo "en_GB.UTF-8" ;;
+        AU) echo "en_AU.UTF-8" ;;
+        CA) echo "en_CA.UTF-8" ;;
+        SG) echo "en_SG.UTF-8" ;;
+        HK) echo "zh_HK.UTF-8" ;;
+        TW) echo "zh_TW.UTF-8" ;;
+        JP) echo "ja_JP.UTF-8" ;;
+        KR) echo "ko_KR.UTF-8" ;;
+        DE) echo "de_DE.UTF-8" ;;
+        FR) echo "fr_FR.UTF-8" ;;
+        ES) echo "es_ES.UTF-8" ;;
+        IT) echo "it_IT.UTF-8" ;;
+        PT|BR) echo "pt_BR.UTF-8" ;;
+        RU) echo "ru_RU.UTF-8" ;;
+        NL) echo "nl_NL.UTF-8" ;;
+        IN) echo "en_IN.UTF-8" ;;
+        *)  echo "en_US.UTF-8" ;;
+    esac
+}
+
+# Query timezone and locale from the current proxy exit IP.
+# Output: <timezone>\t<locale>\t<country_code>
+_geo_detect_tz_lang() {
+    local proxy_url="$1" ip_info detected_tz country_code
+    [[ -n "$proxy_url" ]] || return 1
+    ip_info=$(curl -s --proxy "$proxy_url" --connect-timeout 8 "http://ip-api.com/json/?fields=timezone,countryCode" 2>/dev/null || true)
+    [[ -n "$ip_info" ]] || return 1
+
+    read -r detected_tz country_code < <(
+        echo "$ip_info" | python3 -c "import sys,json; d=json.load(sys.stdin); print(d.get('timezone',''), d.get('countryCode',''))" 2>/dev/null || echo ""
+    )
+    [[ -n "$detected_tz" ]] || return 1
+
+    printf '%s\t%s\t%s\n' "$detected_tz" "$(_locale_from_country_code "$country_code")" "$country_code"
+}
+
+# Validate an IANA timezone name.
+_validate_timezone() {
+    local value="$1"
+    python3 - "$value" << 'PY'
+import sys
+from zoneinfo import ZoneInfo
+
+value = sys.argv[1].strip()
+if not value:
+    raise SystemExit(1)
+
+try:
+    ZoneInfo(value)
+except Exception:
+    raise SystemExit(1)
+
+print(value)
+PY
+}
+
+# Accept a POSIX locale (en_US.UTF-8) or a simple BCP 47 tag (en-US),
+# then normalize to a UTF-8 locale for LANG.
+_normalize_language() {
+    local value="$1"
+    python3 - "$value" << 'PY'
+import re
+import sys
+
+value = sys.argv[1].strip()
+if not value:
+    raise SystemExit(1)
+
+aliases = {
+    "C": "C.UTF-8",
+    "POSIX": "C.UTF-8",
+    "C.UTF-8": "C.UTF-8",
+    "C.UTF8": "C.UTF-8",
+}
+default_regions = {
+    "ar": "SA",
+    "cs": "CZ",
+    "de": "DE",
+    "en": "US",
+    "es": "ES",
+    "fr": "FR",
+    "hi": "IN",
+    "id": "ID",
+    "it": "IT",
+    "ja": "JP",
+    "ko": "KR",
+    "ms": "MY",
+    "nl": "NL",
+    "pl": "PL",
+    "pt": "BR",
+    "ru": "RU",
+    "th": "TH",
+    "tr": "TR",
+    "uk": "UA",
+    "vi": "VN",
+    "zh": "CN",
+}
+
+if value in aliases:
+    print(aliases[value])
+    raise SystemExit(0)
+
+normalized = value.replace("-", "_")
+match = re.fullmatch(r"([A-Za-z]{2,3})(?:_([A-Za-z]{2}|\d{3}))?(?:\.(?:UTF-?8|utf-?8))?", normalized)
+if not match:
+    raise SystemExit(1)
+
+language = match.group(1).lower()
+region = match.group(2)
+if region:
+    region = region.upper()
+else:
+    region = default_regions.get(language)
+    if not region:
+        raise SystemExit(1)
+
+print(f"{language}_{region}.UTF-8")
+PY
+}
+
 # Auto-detect proxy protocol (when user didn't specify http/socks5/https)
 # Usage: _auto_detect_proxy "host:port:user:pass" → returns a working full URL
 _auto_detect_proxy() {