The expression is indeed filer à l'anglaise (backed up by Wiktionnaire).
I don't think there's a specific noun for this. If I were translating something like "Where is he?" — "On French leave" I would just use the past tense: « Où est-il ? » — « Il a filé à l'anglaise. »
In general, a dictionary with a dedicated entry for an idiom should be trusted over Google Translate, in my experience. You really don't know what mélange of contexts the latter has found equivalents in, or if it even knows it should take the two words as a whole.
Also, don't worry about "leave" = congé on its own. Since "French leave" is an idiom, there's no way to guess the meaning from the parts alone. congé + français won't add up to "French leave" and it doesn't have to.