Is the language they are speaking Japanese? If so, why? Did something happen in your setting that -somehow- made Japanese the dominant language? If the language they are speaking isn't Japanese, and Japanese has not been established as a dominant language in the setting I'd probably use another term other then ronin. If they are using a real world language that isn't Japanese, I'd search for a term in that language that means roughly the same thing. If they are speaking a fantasy language, then I'd just invent my own made-up term.
If Japanese is the dominant language, however, then you had better provide a good reason for it. If your story is in "fantasy Japan" then you don't need much, but considering it's a melting pot setting you'll need a reason why all these earth cultures seemingly exist in a totally non-earth world(such as say said fantasy world getting colonized by a collection of culturally diverse earth people thousands of years ago or something.) Now, if it's ACTUALLY earth, but with added magic and/or in the future, and Japanese is the dominant language, you still can't get off without an explanation for why. I have a version of earth like this in one of my stories where both Japanese and English are the most dominant languages. The protagonist(who is culturally American/quite distanced from Japanese culture despite being half-Japanese in terms of genetics.) speaks both English and Japanese, and you can tell when she's speaking which due to subtle grammatical difference and the use of honorifics in speech(to which I plan on providing a glossary/guide for those who don't know how they work and what they mean.).
Now, on this "fantasy/SF/AU earth" Japanese is such a dominant language because the most powerful organization on the planet is a megacorp that was born from a partnership between a powerful Japanese business family and a deranged yet ingenious American inventor who had been living in Japan for some time prior. The corporation itself is an east meets west affair, but the fact that it was born in Japan means that it is directly influenced by that culture and uses it's language.(..I've actually done significant research on various business/corporate structures in Japan specifically with the intent to make this company an accurate depiction of what a "new age zaibatsu" would look like if it existed.)
Now, since this corporation essentially rules the planet at the start of the story, the 'excuse" for Japanese being a dominant language is the sheer power this corporation holds and the influence it has had on politics, business and every sphere of life for many , many years. Due to Japanese being a dominant language in this setting a white person speaking English injecting the term "ronin" into their English speech would not be an -issue- because I have established that Japanese is now a dominant language used everywhere, and in the real world today you see other cultures sometimes using English words or vairations of them in their speech due to it's dominance. Heck, in Japanese you get stuff like doragon, a Japanese spin on the English "dragon"(where as the actual Japanese word for dragon would be Ryū, which while still used to say what it means has a very different connotation then doragon(the latter being the go-to term to describe Europian/western style dragons, among other uses) or tekunorojī which is derived from the English "technology."
Now, if they are speaking in English it's fine to directly interject ronin if you have established Japanese as the dominant language as in English today we pronounce the word "ronin" as "ronin." However, if injecting "ronin" into non-english, non-Japanese speech, you may have to do some more research. As stated with the Japanese examples I gave, while doragon and tekunorojī are derivatives of english words, they are not pronounced or even spelled the same way as their english twins. They are distinctly -Japanese- and thus if the language your characters are speaking in tends to use words from other languages only in this way rather then porting the direct word from the other language you may have to adjust "ronin"'s spelling and pronunciation accordingly. In addition, not every language incorporates words from other languages, even with altered spelling, grammar and pronunciation. If the language they are speaking is one of these, then using "ronin" in any form should not even be considered.
So, whether or not you can use "ronin" either directly or with slight alteration really depends on the setting, what language they are speaking and how it's being used. So tread with caution and be sure to do your research before you throw around the term. Of course, this could also be the "paranoia" label I get thrown at me about my "excessive" desire for accuracy when it comes to these things, so take it for what you will.
