She doesn't speak English. She should not submit a form she transcribes from Russian into English. That can lead to problems in a secondary when someone asks her how she managed to fill out an English-language form, but can't answer English questions.

Two things:

1. Fill out the Russian language form and hand it in.
2. Have with her a card which OP or someone else who reads/writes English and which says, in English, "I am Russian-speaking. I do not speak English. Please find an interpreter. You may also reach my daughter and son-in-law at the following numbers...." I presume that you will be at CVG to greet her and, if called, can simply meet as directed by CBP.

This is a routine issue, it always gets solved. It's harder when it's an obscure dialect of an obscure language. Russian is not that. CBP has easy access to interpreters on the ground, by phone and video.

On a separate note, I would make certain that the MIL has the same card in French in case there are IRROPS at CDG. Again, finding a Russian-speaker won't be that hard, but it's a lot easier for her to walk up to anybody who looks "official" and hand them the card than to have people guessing.

Finally, she should have a mobile with her. There are many other bits & pieces which can go wrong on a long journey with a connection. Far better that she call you than panic.