fix: restore deep dot-notation traversal in Language::getLine()

system/Language/Language.php

@@ -134,7 +134,7 @@ public function getLine(string $line, array $args = [])
     }
 
     /**
-     * @return list<string>|string|null
+     * @return array<array-key, mixed>|string|null
      */
     protected function getTranslationOutput(string $locale, string $file, string $parsedLine)
     {
@@ -160,7 +160,7 @@ protected function getTranslationOutput(string $locale, string $file, string $pa
                 }
             }
 
-            if ($output !== null && ! is_array($output)) {
+            if ($output !== null) {
                 return $output;
             }
         }

tests/system/Language/LanguageTest.php

@@ -381,6 +381,19 @@ public function testLanguageNestedArrayDefinition(): void
         $this->assertSame('e', $lang->getLine('Nested.a.b.c.d'));
     }
 
+    /**
+     * @see https://github.com/codeigniter4/CodeIgniter4/issues/10187
+     */
+    public function testLanguageNestedArrayDefinitionReturnsIntermediateArrays(): void
+    {
+        $lang = new SecondMockLanguage('en');
+        $lang->loadem('Nested', 'en');
+
+        $this->assertSame(['b' => ['c' => ['d' => 'e']]], $lang->getLine('Nested.a'));
+        $this->assertSame(['c' => ['d' => 'e']], $lang->getLine('Nested.a.b'));
+        $this->assertSame(['d' => 'e'], $lang->getLine('Nested.a.b.c'));
+    }
+
     public function testLanguageKeySeparatedByDot(): void
     {
         $lang = new SecondMockLanguage('en');

user_guide_src/source/changelogs/v4.7.3.rst

@@ -46,6 +46,7 @@ Bugs Fixed
 - **Database:** Fixed a bug where the PostgreSQL driver's ``increment()`` and ``decrement()`` methods were not working for numeric columns.
 - **Database:** Fixed a bug where the SQLSRV driver's decrement method was adding instead of subtracting the decrement value when ``$castTextToInt`` was false.
 - **Kint:** Fixed a bug where stale Content Security Policy nonces were reused in worker mode, causing browser CSP violations for Debug Toolbar assets.
+- **Language:** Fixed a bug where ``Language::getLine()`` returned the literal dot-notation key instead of the nested array value when the requested key resolved to an intermediate array three or more levels deep.
 - **Toolbar:** Fixed a bug where the Logs collector raised an undefined property error when using a third-party PSR-3 logger.
 - **Time:** Fixed a bug where ``Time::createFromTimestamp()`` could fail for microsecond timestamps when ``LC_NUMERIC`` used a comma decimal separator.
 - **Validation:** Fixed a bug where ``Validation::getValidated()`` dropped fields whose validated value was explicitly ``null``.