So, there is a problem with FiveM: you can’t Alt+Shift (Ctrl+Shift) to switch OS language: once you’ve launched the client, it’s locked. Even if you Alt+Tab, switch in Windows and go back to game, it switches back to what it was.
Yes, I consider it as a bug, but seems like devs do not, and I’ve seen servers which have ability to change the language in game already. So, I guess, there is a workaround.