let getLocalizedPath on the i18n service return a promise.

addon/utils/t.js

@@ -26,12 +26,21 @@ function T(attributes) {
 
     locale = service.resolveLocale(this.container, this);
     result = service.getLocalizedPath(locale, path, this.container, this);
-    result = service.applyPluralizationRules(result, locale, path, this.container, values, this);
 
     Ember.assert('Missing translation for key "' + path + '".', result);
-    Ember.assert('Translation for key "' + path + '" is not a string.', Ember.typeOf(result) === 'string');
 
-    return service.fmt(result, readArray(values));
+    var self = this;
+    if(Ember.typeOf(result) === 'object' && Ember.typeOf(result.then) === 'function') {
+      return result.then(function(val) {
+        val = service.applyPluralizationRules(val, locale, path, self.container, values, self);
+        Ember.assert('Translation for key "' + path + '" is not a string.', Ember.typeOf(val) === 'string');
+        return service.fmt(val, readArray(values));
+      });
+    } else {
+      result = service.applyPluralizationRules(result, locale, path, self.container, values, self);
+      Ember.assert('Translation for key "' + path + '" is not a string.', Ember.typeOf(result) === 'string');
+      return service.fmt(result, readArray(values));
+    }
   };
 }
 

app/helpers/t.js

@@ -8,22 +8,42 @@ export default function tHelper(params, hash, options, env) {
   var t = container.lookup('utils:t');
   var application = container.lookup('application:main');
 
+  var cache = "";
   var stream = new Stream(function() {
-    return t(path, params);
+    return cache;
   });
 
+  var update = function() {
+    var tRes = t(path, params);
+    if(typeof(tRes) === 'string') {
+      cache = tRes;
+      stream.notify();
+    } else if(typeof(tRes) === 'undefined') {
+      cache = '';
+      stream.notify();
+    } else if(typeof(tRes) === 'object' && typeof(tRes.then) === 'function') {
+      tRes.then(function(val) {
+        cache = val;
+        stream.notify();
+      });
+    } else {
+      throw 'unexpected type returned from t util';
+    }
+  };
+  update();
+
   // bind any arguments that are Streams
   for (var i = 0, l = params.length; i < l; i++) {
     var param = params[i];
     if(param && param.isStream){
-      param.subscribe(stream.notify, stream);
+      param.subscribe(update, this);
     };
   }
 
-  application.localeStream.subscribe(stream.notify, stream);
+  application.localeStream.subscribe(update, this);
 
   if (path.isStream) {
-    path.subscribe(stream.notify, stream);
+    path.subscribe(update, this);
   }
 
   return stream;

tests/unit/utils/t-test.js

@@ -235,3 +235,44 @@ test('can override the format handler', function(assert) {
 
   assert.equal(t('foo'), 'barbiz');
 });
+
+
+test('locale lookup handler returns promise', function(assert) {
+  define('dummy/services/i18n', ['ember-cli-i18n/services/i18n'], function(s) {
+    s['default'].getLocalizedPath = function(locale, path) {
+      return new Ember.RSVP.Promise(function(resolve) {
+        resolve("foo-bar");
+      });
+    };
+    return s['default'];
+  });
+
+  application.defaultLocale = 'en';
+
+  t('foo').then(function(v) {
+    assert.equal(v, 'foo-bar');
+  });
+});
+
+test('promise pluralization', function(assert) {
+  define('dummy/services/i18n', ['ember-cli-i18n/services/i18n'], function(s) {
+    s['default'].getLocalizedPath = function(locale, path) {
+      return new Ember.RSVP.Promise(function(resolve) {
+        resolve({
+          one: '%@ friend',
+          other: '%@ friends'
+        });
+      });
+    };
+    return s['default'];
+  });
+
+  application.defaultLocale = 'en';
+
+  t('friend', 1).then(function(v) {
+    assert.equal(v, '1 friend');
+  });
+  t('friend', 5).then(function(v) {
+    assert.equal(v, '5 friends');
+  });
+});