The first step to doing this will require me making rather large changes to the app to accommodate the language changes. I probably should have done this from the beginning, but as I noted in the description of the app, I'd originally written it just for myself, but then later decided to post it on Google Play to let others use it.
As for street names being mispronounced by TTS engines, I'm not sure how a change in the app will help there. All directions are generated by RideWithGPS (where you can edit the exactly phrasing of the turn instructions). These instructions are then passed to the TTS in exactly the form they take in the file generated by RWG. I guess if you used a non-English TTS, the other wording coming from the app might not be said correctly, but most of the phrases that the app uses in directions can be changed in the "Turn by Turn Tweaks" section of the Settings. These don't cover absolutely everything, but if users can tell me the phrasing that presently CANNOT be customized in Tweaks, then let me know and I'll compile a list of them, and then add new phrases to the Tweaks section.