I definitely think that at first Reid was only an expert in the English language.
But Reid started mentioning having at least some passive skills in a few languages (being able to listen and read, but not speak much), watching Russian and French movies etc, in about season 5 (?).
This actually makes sense and is realistic, as he would be able to teach himself those skills quickly by listening to audio books (which we know he loves) in combination with versions of the same book in English and the other language. Add in some DVDs with subtitles to learn colloquial expressions and an exceptional memory, and this would be completely possible.
He possibly would only be able to say a couple of expressions in each language, as passive and active skills and often not anywhere near the same level even in your native language.
Don't get me started on speed reading War and Peace, though. Speed reading just doesn't work for novels if you are trying to enjoy them and not just look for information.