Should nouns borrowed from Japanese be pluralized?
As someone who has watched a lot of subtitled Japanese animation, it seems odd to hear a word such as ninja (used in the plural) in the dialogue and see it transliterated as ninjas.
It somehow seems better to me to treat ninja just like antelope, bison, buffalo, caribou, deer, elk, fish, grouse, quail, reindeer, sheep, swine, etc., which are both singular and plural.
Would you also insist that Japanese speakers pluralize English words when used in the plural?
Once a word has been borrowed into a language, it adheres to the grammar for normal words in that language. We don't borrow Japanese grammar, just words, so there is no need to use a zero plural with borrowed Japanese words.
It is true there is a small set of animal nouns in English that have a zero plural, but they are not borrowed and are a special case.
This is borne out by the results in the Corpus of Contemporary American English, which has 68 incidences of "ninjas". Of the 496 incidences of "ninja", there are a handful of uses of "ninja" as a zero plural, as OP suggests, from the script for the 2003 film The Last Samurai, but the vast majority are singular (or attributive) usage. So it does appear that zero-plural "ninja" is used, albeit uncommonly so.
So yes, ninja meaning ninjas is a usage that gets some use, but regular pluralized ninjas is more common, and perfectly grammatical.
English has always had zero plurals as one of many types of irregular plural along with the regular plurals. For both borrowed and native words. So do related languages including German.
Neither transliteration nor Romance has a lot to do with it since we have nouns with both regular and irregular plural forms borrowed from Romance languages and other languages in the same alphabet and other alphabets.
In the case of ninja, it comes from Japanese and Japanese, like many languages, does not inflect for plural. Others are Chinese, Korean, Thai, Vietnamese, Indonesian, Malaysian, and all Polynesian languages including Hawaiian and Maori.
For Japanese it seems to relate to how mainstream a borrowed word has become in English. If it doesn't yet have wide currency or refers to something exotic it has a higher chance of retaining the Japanese (zero) plural. Here are some English words borrowed from Japanese in their plural form the way I say them, others may disagree:
- anime (though animes doesn't sound wrong)
- bonsai (perhaps uncountable in English)
- futons (never futon)
- geisha sounds better than geishas to me
- hibachi isn't common in Australia but I assume Americans say hibachis
- judo doesn't have a plural
- kabuki (never kabukis)
- kanji sounds right to me - I always cringe when I read kanjis
- karaoke (perhaps uncountable in English)
- karate doesn't have a plural
- kimonos sounds better than plural kimono to me
- manga (perhaps uncountable in English)
- miso is definitely uncountable in English
- origami (perhaps uncountable in English)
- otakus sounds better than otaku as a plural to me but this word hasn't gained much currency in English as yet
- ramen (never ramens)
- sake is uncountable in English
- samurai sounds slightly better than samurais to me
- sashimi (never sashimis)
- shiatsu (has no plural in English)
- shinkansens sounds wrong but so does plural shinkansen!
- shinto doesn't have a plural
- sumo doesn't have a plural
- sushi (never sushis)
- tempura seems to be uncountable in English
- tofu (never tofus)
- torii (never toriis, has a similar form to some Latin plurals)
- tsunamis (never tsunami)
- wasabi is uncountable in English
- yakuza (though yakuzas doesn't sound wrong)
- yen (never yens, which is another one which makes me cringe when I hear it)
- zen (has no plural)
We do this all the time with Latin and Greek loan-words in English. (The former is technically not a Romance language, but rather the progenitor of them.)
e.g. Octupus -> Octopodes (Greek), when we typically use "Octopuses"
e.g. Peninsula -> Peninsulae (Latin), when we typically use "Peninsulas"
There are many, many more examples, in particular of Latin origin.
Generally, because these words have been around so long in the language, we anglicise their plural forms, rather than keeping the originals, out of convenience (ignorance?). Some will always insist to use the original Latin/Greek plurals, of course. With more recent loan-words, it is more common to keep the plural form of the original language.
My view is that we should use the plural form of the source language when known/in common use, and anglicise the word by adding -s or -es otherwise. Each to their own, however.
I think the pattern is that English speakers pluralize with an "S" when in doubt, and some of the plurals that sound doubtful in English end in "A" or "I". As another example, I hear "schemas" on a daily basis, but never heard "schemata" except when I tried it myself a few times.