Japanese doesn't do plurals like that, but English does. And when a Japanese word is put into an English sentence, it becomes a loanword, and so may follow the rules of English grammar along with all the other words.

(If you're telling me that we Swedes can't say "sandwichar" because it's an English word so we should put English plurals on it, or for that matter if we can't put a definite suffix on it because English doesn't do that, it would make many Swedish sentences extremely cumbersome. I think that when an English word becomes part of our language, it in effect turns Swedish and should follow the rules of Swedish grammar. And I think it's the same with Japanese word in English (at least in principle: in practice I admit it varies depending on words which ones I'm comfortable using plurals for, likely influenced by what I've seen from others).)