To give some examples:
He has a certain je ne sais quoi about him. (French)
I thought I'd go au naturel for our girls' night out. (French)
That skirt with that blouse, I must say, what a fashion faux pas. (French)
I don't like my meat al dente, I prefer it well-done. (Italian)
Your meshugge cousin can be quite the klutz! (Yiddish)
Oy vey, that kamikaze klutz almost gave me a heart-attack! (Yiddish, Japanese)
My question here is, is it grammatically correct to use more than one language in a sentence?
I know it is accepted to do so (appropriately). But! if anything this site in particular has taught me, just because it is accepted, it doesn't mean that it is correct.
Reference(s) would be nice.
There is a comment regarding au naturel meaning naked, and while that is one definition, I'd like to point out that it also means
in a natural state