Does the word polyglot, meaning "knowing or using several languages" translate directly into German, or is there an idiomatic equivalent? Google Translate just adds an extra "t" at the end, as German doubles consonants.
Polyglott is correct, but it's not widely known. You could use that, but the word mehrsprachig (which is the same), is more common, so I'd recommend using that.
In addition to looper's answer, you could also say vielsprachig/mehrsprachig.
This next part is only because you said idiomatic. If you change your thinking a bit, you could say, "he has a talent for languages."
- Er ist sprachbegabt.
- Er hat eine Begabung für Sprachen.
It would be implied, although not necessarily true (as looper pointed out in the comment below), that he can speak several languages. I should also note that "begabt" implies an ability given to you e.g. "He has a God-**given* * ability."
In addition to "sprachbegabt/vielsprachig" there is also "Sprachtalent" if you are looking for a noun equivalent, which may make for more straight-forward translations.