Add supported_languages parameter to STT and TTS base classes (#3059)

* Add `supported_languages` parameter to STT and TTS base classes

* Refactor `supported_languages` to `available_languages` and updated docstring to be more precise

mycroft/stt/__init__.py

@@ -37,6 +37,18 @@ def __init__(self):
         self.recognizer = Recognizer()
         self.can_stream = False
 
+    @property
+    def available_languages(self) -> set:
+        """Return languages supported by this STT implementation in this state
+
+        This property should be overridden by the derived class to advertise
+        what languages that engine supports.
+
+        Returns:
+            set: supported languages
+        """
+        return set()
+
     @staticmethod
     def init_language(config_core):
         """Helper method to get language code from Mycroft config."""

mycroft/tts/tts.py

@@ -193,6 +193,18 @@ def __init__(self, lang, config, validator, audio_ext='wav',
         )
         self.cache.clear()
 
+    @property
+    def available_languages(self) -> set:
+        """Return languages supported by this TTS implementation in this state
+
+        This property should be overridden by the derived class to advertise
+        what languages that engine supports.
+
+        Returns:
+            set: supported languages
+        """
+        return set()
+
     def load_spellings(self):
         """Load phonetic spellings of words as dictionary."""
         path = join('text', self.lang.lower(), 'phonetic_spellings.txt')