You would use software like Word, and create three columns. The first column has a key (which software like Unity use), the second column has the English text and the third column has the translation. Like this:
| Key | English | Russian |
| NewGame | Start New Game | Начать новую игру |
I don't know anything about Ren'Py, but with Unity it's complicated to insert a translation after the game is complete. So you'd be better off offering your service to someone still developing a game, or who already has translations in their game.
Just randomly translating a game might be pointless because you can't publish it, (it's copyright infringement) and the developers might not be able to incorporate it into their original game.