There is no multi-language support as far as I know. Firebase gives you database and you can prepare your data model to support different languages. As a example, you could have just a key in every data where a text is, and then you could use different node like (/transaltion/$language/$key - i.e. /translation/en/title) to provide proper translation. This is maybe not the best way, but illustrates how you can do it. If you don't want to fetch data for proper language every time, you can prepare your data that you can fetch proper language upfront (in app startup) and cache translated text for proper $language.
cheers,
p.