[denton-scratch]

Tzatziki, Tsetse and Tzar are not English words; OP said there was just one English word (which he identified as "pizza", which isn't an English word).

[xnorswap]

It has foreign roots but it's surely an English word by any workable definition.

[eesmith]

And "tsetse" has been used in English for nearly 100 years longer than pizza - the latter only really introduced because of Allied soldiers sent to in Italy.

Google Ngram says it wasn't until the 1970s when "pizza" was more commonly used than "tsetse." https://books.google.com/ngrams/graph?content=tsetse%2Cpizza... . Or from BoingBoing today, "North Americans feared and misunderstood pizza in the 1950s" - https://boingboing.net/2022/11/21/north-americans-feared-and... .