This is just a personal opinion, I do not mean any offense because I am thinking about work aesthetic.
I think using a full romanized sentence is the worse method to go to. Even if it is just one or two, it would give the effect that the author is not doing the research as they should have, thus giving the impression they are not that invested in the story. Sure, it would be a good idea but it doesn't look pleasing.
If you're going to use romaji (which will look like a sentence that won't make sense unless you translate them), then might as well use Kana and Kanji. For kanji use furigana for reading aid (to make it look a bit more professionally done). Then on the same speech bubble, give the translation in brackets. Translation machine is barely reliable but for one sentence or two, they will make do.
But I am still leaning towards using different fonts for different language. You wouldn't need to necessarily rack your brain or unconsciously offending a small group of ethnics because you don't understand the language and culture and readers would still understand that they are talking in a different language.