Yes, standard voices are used in conjunction with standard language. That's why the English lang file works with your audio files.
In order for the TTS lang file to be used, you must also have the corresponding TTS voice file, the TTS engine must be installed and set as the system voice in the device instead of Google's default.

Write PM to Titanic, he is a good person and he will gladly help you.
I think that you will communicate the easiest in your language