Settings: change the TTS text to use the current locale

Google TTS currently does not support language auto-switching.
Also, not all languages are available in TTS at all times.
This means that TTS cannot properly handle text in other languages / scripts
than the current language.
Languages with different scripts would not be read at all, and the others
would be read with "bad pronunciation", to the point of being unrecognizable.

This change is not a full fix, but makes things slightly better by using
the default locale for the contentDescription in the language / region lists.

This might be a bit unsettling, as the label will not match the sound.
The label will show "Deutsch" or "Русский" and text-to-speech will say "German"
or "Russian" (for English UI), or "Allemand" or "Russe" (if the UI is French).
Might also say nothing, if the current UI language has no text-to-speech engine.

But this is the best we can do, and better that what we get if we do nothing.

Bug: 28087937
Change-Id: I54d49ec96a10bab3e31198e162230fb277426e66