You can, but I would advise not just stopping there and moving on. The term "fluent" can be quite a broad one for us linguists. One person’s definition of fluent may not be another’s. I would advise going a bit further and checking the candidate can do specific tasks in that foreign language, related to the job you are recruiting for.
Even myself as a linguist, I struggle to answer when people ask me if I’m fluent in Russian or French.
I prefer the question: How well do I know these languages? What level am I at with them? Could I conduct a meeting in the language? How comfortable am I speaking over the phone in Russian? Etc.
You get the idea. You will get a better idea of a candidate's language skills by asking these types of questions rather than just asking ‘Are you fluent?’ then moving on.