It will automatically switch the language depending on your current language code. How do you set your language code? Well, it's set automatically by Telegram. I think when you change the interface language in Telegram, it changes the language in LibreTaxi. Sometimes you need a couple of minutes for changes to take effect.
So now technical details if you want to translate it from the very beginning. Look at this text:
https://github.com/ro31337/libretaxi/blob/master/locales/all/en.po