fix(pocket-tts): honour the language option when loading the model (#12766)

Pocket TTS ships one model per language, but LoadModel called
TTSModel.load_model() with no arguments, so the library always fell back
to its English model whatever the model config said. Read a `language`
option and pass it to load_model, keeping the library default when it is
not set. Document the option and add unit tests that mock the loader.

Fixes #12759

Assisted-by: Claude:claude-opus-5-5

backend/python/pocket-tts/backend.py

@@ -102,9 +102,17 @@ class BackendServicer(backend_pb2_grpc.BackendServicer):
         self.default_voice_url = self.options.get("default_voice", None)
         self._voice_cache = {}
 
+        # Pocket TTS ships a separate model per language (english, french,
+        # german, ...) and load_model() falls back to English when no language
+        # is given, so the option has to reach the loader to pick another one.
+        load_kwargs = {}
+        language = self.options.get("language")
+        if language:
+            load_kwargs["language"] = str(language)
+
         try:
-            print("Loading Pocket TTS model", file=sys.stderr)
-            self.tts_model = TTSModel.load_model()
+            print(f"Loading Pocket TTS model (language: {load_kwargs.get('language', 'default')})", file=sys.stderr)
+            self.tts_model = TTSModel.load_model(**load_kwargs)
             print(f"Model loaded successfully. Sample rate: {self.tts_model.sample_rate}", file=sys.stderr)
 
             # Pre-load default voice if specified

backend/python/pocket-tts/test.py

@@ -6,6 +6,7 @@ import subprocess
 import time
 import os
 import tempfile
+from unittest import mock
 import backend_pb2
 import backend_pb2_grpc
 
@@ -139,3 +140,33 @@ class TestBackendServicer(unittest.TestCase):
             self.fail("TTS service with default voice failed")
         finally:
             self.tearDown()
+
+
+class TestLanguageOption(unittest.TestCase):
+    """
+    Checks that the language option reaches TTSModel.load_model without
+    downloading any weights.
+    """
+    def _load_model(self, options):
+        import backend
+
+        fake_model = mock.MagicMock()
+        fake_model.sample_rate = 24000
+        with mock.patch.object(backend.TTSModel, "load_model", return_value=fake_model) as load_model:
+            response = backend.BackendServicer().LoadModel(
+                backend_pb2.ModelOptions(Options=options), None
+            )
+        self.assertTrue(response.success, response.message)
+        return load_model
+
+    def test_language_option_selects_model(self):
+        load_model = self._load_model(["language:french"])
+        load_model.assert_called_once_with(language="french")
+
+    def test_versioned_language_option(self):
+        load_model = self._load_model(["language:english_2026-04"])
+        load_model.assert_called_once_with(language="english_2026-04")
+
+    def test_no_language_keeps_library_default(self):
+        load_model = self._load_model(["default_voice:azelma"])
+        load_model.assert_called_once_with()