I've seen plenty of fanfiction that gives the reader a key to know what is being said how. This does not, and probably will never, fly in professional writing. But... I haven't read much where multiple languages, and varying levels of understanding, are a part of the story and plot, and thus I haven't read much of how it's been edited for "easier" reading. I've seen some italics used, a little bit more of things like signed instead of said, but that's about it. Early drafts it's fine to do whatever, that's the author keeping it simple for themselves, but what about when it comes time to shop it around to get it published? Are there established best practices for formatting?
Here's an article about how to incorporate invented languages into a fantasy novel. Some of the information, I think, is applicable to incorporating real, other languages, as well. 5 Ways To Incorporate Multiple Languages Into Your Fantasy Novel
I find it best to summarize what is being said in other languages and who understands what than actually using another language in dialog. Sure, a word or two might be fine if they are pretty well known, but even then I don't think actually using the other languages adds much to a story. This is something I've played around with quite a bit. Clarity is king. I think that's important to remember and should remain the focus when dealing with characters who speak other languages.
Just do what Cormac McCarthy does... or did. Write everything in the other language with no translations or attempts at clarity. If you can't understand it, tough beans! I love that guy. Rest in peace.
For my fantasy novel I use a combination of italics to indicate a non-standard language and mentioning what language what those are in, as well as something describing the dialogue indirectly if the character's understanding is rough or nonexistent. I also include some words or phrases from different languages where appropriate, particularly my main conlang that I'm actually created in any fullness. The fun part about the italics method is I change it depending on what the standard language is. The standard language is always translated to English and written in standard, unedited, font, but it may be different for different characters. While for my other four main characters the standard language is Gethermen, a language that is linguistically and culturally similar to English, my character Latirah's standard language is a more Afro-Asian language called Nadisey and Gethermen is italic-coded as "foreign" when she hears or speaks it, even though she is fluent. So there's some implicit commentary on norms there.
I wish I had the nerve to do that in my W.I.P. I'd let rip with German where characters are speaking that, and English where that's what they're speaking. But in romantic suspense I doubt it'd fly.
Author Junot Diaz is a master of Spanglish, artfully blending English in Spanish in his prose. His novel "The Brief Wondrous Life of Oscar Wao" is a prime example of this. The way he does makes me feel like I would understand the text even if I didn't know Spanish. You can definitely check his short stories for some inspiration.
