Just off the top of my head, depending on accent or dialect: "spritzer", "waltzer", "Mitsubishi", "matzos", "mitzvah".

A fun code challenge! I started with https://github.com/Alexir/CMUdict/blob/master/cmudict-0.7b as a list of words with pronunciation. It has:

  PIZZA  P IY1 T S AH0

Looks like I can search for " T S " to find the "ts sound that's not at the end"

  % grep " T S " cmudict-0.7b | wc -l
       991

Some of the non-proper nouns include ACCOUNTANCY, ANTSY, and ARTSY. I'll assume this is too close to the end to count, so require two sounds after the " T S ":

  % grep -E " T S [A-Z0-9]+ [A-Z0-9]+" cmudict-0.7b | wc -l
       853

These include: bestseller, blitzkrieg, boatswain, bootstrap, chutzpah, craftsman, draftsmanship, footstep, hotspot, itself, jetstream, outscore, outsell, outside, postscript, shirtsleeve, shortstop, sportsmanship, statesmen, tsar, tsunami, whatsoever, yachtsman, and zeitgeist.

(It doesn't have "spritzer" in the list, and uses the spelling "matzoh" instead of "matzo". I didn't look for a more complete list of word pronunciations.)

Finally, two words with two occurrences of the "ts" sound, neither at the end:

  % grep -E " T S .*T S " cmudict-0.7b
  ITSY-BITSY  IH2 T S IY0 B IH1 T S IY0
  TSETSE  T S IY1 T S IY0

"Zeitgeist" seems like a bug, I've never heard anyone pronounce the leading 'z' as 'ˈts' in English, and doing so as a non-German might even be seen as 'over-pronuciation'.

https://en.wiktionary.org/wiki/zeitgeist#Pronunciation gives two pronunciations:

Pronunciation (Received Pronunciation, General American) IPA(key): /ˈtsaɪtˌɡaɪst/, /ˈzaɪtˌɡaɪst/

https://www.merriam-webster.com/dictionary/zeitgeist also gives two: ˈtsīt-ˌgīst ˈzīt-

I tried to get statistics from listening to https://youglish.com/pronounce/zeitgeist/english? but found I have a hard time distinguishing the two forms.

"boatswain" should be pronounced "bosun".

You're right. Though https://en.wikipedia.org/wiki/Boatswain says "formerly and dialectally also /ˈboʊtsweɪn/ BOHT-swayn", the dictionaries I checked do not list that alternative.

https://www.etymonline.com/word/boatswain even comments:

> Phonetic spelling bo'sun/bosun is attested from 1840. Fowler [1926] writes, "The nautical pronunciation (bō'sn) has become so general that to avoid it is more affected than to use it."

Most of the examples you gave are compound words, or made through suffixes and who knows, maybe my Russian teacher told a lie!

It certainly seems your Russian teacher wasn't fond of schnitzel nor seltzer.

I was literally laughing so hard from this comment. Thanks for brightening my day.

Seems implausible.

~ Fatso

~ Pitstop

~ Katsup

~ Yachtsman

~ Scotsmen

~ Batsman

There don't seem to be many, admittedly.

That fact seems dubious, does tzatziki not count?

Is tzatziki the only word that both starts and contains, but doesn’t end, with ts sound?

tsetse. https://en.wikipedia.org/wiki/Tsetse_fly

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).

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

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... .

That is trivially not true.