If you're in 1st person or 3rd person limited narration and the narrator character does not understand the language, put it in the original language. If it's full dialogue with 3rd person omniscient or the narrator character does understand the language, I'd say translate into the book's language as closely as possible.
Basically the idea is that the audience should understand as much as the character observing the dialogue does. Barely anyone is going to look up translations, especially if there's more than one line in that different language, they'll give up on the translations and possibly even the book if it gets too frequent. The only way to make that work is if the narrator character also does not understand so they can still read the book without feeling left out.
I've never really seen it done any other way in novels, in comics the best I've seen is Stand Still Stay Silent where language is a big part of the book's theme so a lot of page time is dedicated to telling the audience which characters speak which languages (almost all of them being multilingual) and what language they're speaking is shown with a little flag in their speech bubble. If multilingual people are going to be a big part of your novel, spending time saying who speaks what and how well the others understand can be a good thing to note, and then occasionnally saying " "bla bla bla" he said in french."