All game texts are stored in a special archive called tetxts.pak (excluding custom text files created from scratch). Personally, I would do it this way: by default, the map has one voiceover (e.g., Russian), and to enable another, you download a mini-mod that also contains text files, but in English. And with a simple trick, the Russian language turns into English. But this is if you just want to make the map bilingual (without dynamic switching). You can start the map and display a talkbox, asking the player which language they prefer, and then use a special command, OverrideObjectTooltipNameAndDescription, to change the names and descriptions of objects. But this requires a lot of scripting. However, the main point is: I have no idea how this will work with 2 players. Someone else who is familiar with this is needed.
Будь яростней пред ночью всех ночей,
Не дай погаснуть свету своему!
Хоть мудрый знает – не осилишь тьму
Во мгле словами не зажжёшь лучей –
Не уходи безропотно во тьму.