All
ingame texts are switchable to a bunch of languages including russian, polish, german, turkish. If you're on Linux, you can also have Chinese (not available on windows for technical reasons).
This is done by clicking on the flag button on the small menu at the top. Not very discoverable, sadly.
Some languages are partially translated and not included due to lack of completion - Spanish is at around 20% total, for example. You can pretty much start translating this whenever you want.
Lobby and campaign are not translated nor currently translatable. If you want to do either, i'll tell you where to start - it's rather simple lua stuff mostly, but there's lots of it.