I don’t perceive “Quesadilla” as being foreign at all. It’s just as English as any other English word.

Some words that are fully considered English have an even more convoluted history than this. The word for “chocolate” came from Spanish, and came to Spanish from Nahuatl which is completely unrelated to English, and may (though it gets obscure here) have in turn come to Nahuatl from an unrelated Mayan language.