In a French restaurant that might be a faux pas, but in American English "au jus" is thought of as a side sauce, not its literal French meaning. If you are at a deli and the waitress asked you "would you like jus, with your sandwich?", you would think her an idiot or pretentious. She is going to ask, "would you like au jus, with your sandwich?" You can buy packets of "au jus" in any supermarket in the U.S., you are not buying a packet labeled "jus". If a waitress knows enough to ask whether I want something with jus, she'll ask whether I want it 'au jus'; this isn't idiotic or pretentious, it's acknowledging a well-recognized convention. I haven't seen packets of any substance marked 'au jus' in US supermarkets; where does this happen? Mass misuse doesn't make something okay. If someone can't wrangle terms in other language, better to stick with what they know; there's nothing so damn special about being multilingual, so attempting it, only to fail, is silly. It is not a well recognized convention, the well recognized convention is that "au jus" is a sauce in the U.S. so asking someone if they want the sandwich "with au jus" would be the correct, idiomatic U.S. usage. You can buy Lawry's, or McCormick's or your store brand (Krogger, Safeway, etc.). Here is an example from Red Robin. Their prime rib dip comes "with au jus", which is the correct, idiomatic American English usage.