Looking at this question, I was trying to figure out how to format a small family of novels and short stories that use a number of languages. My original approach went with the above question's style. However, over the last few years, I've been to a number of panels about multiple languages and had conversations where many native speakers feel that setting off foreign words in italics came off as jarring when they are usually smoothly integrated into their speech. Given that, they recommend that foreign words were not italicized.
"No," said Chimípu, "I don't think so. When we were fighting, she used the tazágu and I didn't see any knives. Whoever killed Karawàbi used a straight blade; the wound is too clean and deep."
Using the above example, the POV character (Rutejìmo) is a native speaker of the language, so tazágu wouldn't be italicized.
Which seemed like a reasonable thing until I hit the bilingual character who initially doesn't know the foreign words:
Her mother raised an eyebrow and looked at the chest. "You had a sands damned bag you packed yesterday. It was small and light, worthy of travel. What is that fupujyu chìdo? Are you taking that instead?"
In this case, "fupujyu chìdo" would fine italicized or not, it is jarring since the main character (Kanéko) doesn't know the words but the untranslated word could be either.
However... sands damned is notionally translated (translated into English since the POV character knows the word). I wanted to demonstrate her mother alternates frequently between two languages (and swears up a storm but that's the basis for her magic). Later in her series, she gets fluent between two languages (much like her mother) so there is a lot more notionally translated phrases and entire sentences.
To add special complexity, I'm planning on combining these two novels together (well and all the other stories) into a single book to a master plot across multiple novels. That means native speakers (which I think would be better not italicized to keep them in harmony), with unknown words, and notionally translated at the same time. Plus, there are nine or so other languages across the combined novels though never more than three in a given chapter (because it gets annoying).
I thought about translating it after the phrase, but there is a lot of speaking in these novels and there some characters who bounce between two languages constantly (mainly because I grew up with a polyglot household so that was the norm for me).
So, does the attempt to not italicize for native speakers make sense? Would have it not italicized for native points of view but italicized for non-native be reasonable?
Would indicating the native language in the chapter headers for the combined one solve the problem? I could do the same at the beginning of the book.
I'm not sure about the triple language chapters though. Originally I thought of using three separate fonts but I wasn't sure if that made sense. In the first version, I used serif for native, italicized serif for the second, and sans serif for the third language.