Technically, RSVP is a French term, or to be more exact, an abbreviation for repondez vous s'il vous plait. It seems to have found its way into the English language as a fairly common term.
Are questions about it on topic?
What about other terms with foreign origin such as faux pas or savoir faire? Where does one draw the line?
A day after asking the original, I saw this question about "alpha" and "omega," that was closed:
The (original) Greek term is occasionally used to refer to "beginning and end." It was used in the above context to refer to "birth and death." That's not exactly a common English usage, so it a bit of a stretch, but is understandable from the Greek usage.
Is it OK if an expression is USABLE in English? Or does it have to be in "common use."
Is that going too far?