In the US today, I have the luxury of being able to eat at a variety of "ethnic food" restaurants (with varying degrees of authenticity, admittedly).
I can decide to have Austrian, Thai, Italian, Salvadoran, Ethiopian, etc.
At what point in history did this become "a thing"? Did Roman soldiers go out after a long night drinking to try that new Egyptian joint they'd heard so much about? Did the Sumerians have a hankering for Akkadian grub for brunch?
Note that I'm not asking about the cultural penetration of individual food items. The fact that you can get spaghetti at Denny's doesn't make it an Italian restaurant. I'm talking about restaurants that are pointedly of a cuisine other than the 'norm' for the host culture.
Follow-on question: what are the earliest records of restaurants at all?
To find the earliest ethnic restaurants one must first find the first places we have proof existed that fit the definition of restaurant. Thermopolium Picture Linked fit the bill as well over a hundred have been excavated from the ancient Roman city of Pompeii alone. The setup was simple, with an L-shaped bar inset with large vase-like holes in which food was kept to be served to customers.
[Edited] Remember there were many more languages in the world during this time period than there are today. "The Common People of Ancient Rome" states that even in nearby towns like "Rome, Tusculum, and Praeneste...although Latin was spoken in all of them it varied from one to the other" (Abott, 5). Thus depending on what period of Roman history is studied 'foreigner' could mean anything from the next city over (Around the founding of Rome), to someone outside of central Italy, to Italy itself.
It is therefore highly likely that some of the hundred plus Thermopolium found in Pompeii were operated by those who Pompeiians, if not Romans, would consider foreigners. Thus the oldest ethnic restaurants for which we have evidence (Physical and text) are quite likely to be from this time period.
