So the question of whether you should include foreign languages in your fiction comes down to whether you have a reason to do it at all. And no, ambience is not a reason.
As a rule of thumb, the answer is just don’t.
First of all, every word you write in a foreign language is a word you’re writing blind (obviously this is not the case if you’re a fluent bilingual) trusting another person to have understood what you meant and translated the nuance properly. Alternative: use “they said in Russian” or establish certain characters speak in one language to each other, then stop mentioning it until a 3rd character who doesn’t speak Russian walks in and it becomes noticeable. Or until your MC forgets how to say ‘watermelon’ in English and has to describe it to the MC2 (when doing something like that, beware of stereotypes associated with the other language, it’s a very real experience but characters are representations of real people and you don’t want to be pointlessly offensive).
Second, your reader is probably not fluent in that language and you’re risking them missing a lot of nuance. Again, if you’re writing about your own bilingual community and this is a big theme in your book, solid reason, keep going, though I beg you not to repeat every sentence you write in Italian in English and use context and body language to fill in the gaps for non-bilingual readers, otherwise you’re boring your bilingual readers just to have a lot of nonsense words for those who don’t speak the language.
With ebooks, going back and checking the index for translations is particularly cumbersome and it will disrupt the flow of reading regardless.
For bilingual/bicultural writers:
Is biculturalism or bilingualism a theme in your book? Is it something that you want to explore? Or have as a background fact?
Sometimes I do one, sometimes I do the other. Just know where you’re going. If the other language has meaning, then you’re asking the reader to make an effort to keep track of it for a reason.
In The Stars of the Pack series, but particularly in "Simpler Than Most" and later in "Beloved of the Pack", the relationship between Iesu and Sergi starts off as casual and Sergi's efforts to learn Romanian is an declaration of love and commitment as well as a visible sign of their intimacy since they can speak to each other privately even when surrounded by their pack. They also share a recurring phrase meaning 'Shut up' (checked with a native speaker, naturally) because Iesu can be a bit cheeky. Later on, in "Beloved of the Pack" other characters in the pack learn this phrase, deepening the connection between them all. Romanian is also the second language all of the pack children learn, regardless of who their biological father is, in this way, the pack bonds are further strengthened through this common view of the world that is another language.
By using this single line of Romanian repeatedly, I easily established its meaning for non-Romanian speakers, who could then simply associate the language with 'shut up' and their playful back and forth. On occasions when it was relevant, I clarified that a character was speaking Romanian. When the narrator is Ray, who doesn't understand it, it's something he will notice, but if the only people in the scene are Iesu and his cousin Irina, they're likely to be speaking their native language but they're also likely to be mixing in some English words and in either case, it is neither here nor there unless one of them decides to make it an issue (to tease, correct, etc).
Take it from me, on any given day, I could not tell you what language I have last spoken to my sister if you paid me; once you're fluent, it simply becomes another way to communicate unless you're working on something language related like grammar or poetry.
Interesting language scenarios:
* One character is less fluent.
* MC2 is learning it to speak to MC1 and it works as a reminder of their devotion.
* MC1 is in a foreign culture and language is the one aspect they can navigate but it’s not quite enough to understand the people.
* MC1 and MC2 have a row about colours because their languages make them perceive the concepts differently...
* Or philosophy, what about languages without gendered pronouns do to the way you think of yourself and others?
* Or mysticism, such as the way some cultures & languages will not mention dead people’s names, or use metaphors and euphemisms for certain concepts?
If you are a writer, you already love language, adding an extra one can be tempting... but you’d not pretend play the flute, why would it be okay to pretend speak Polish?
As usual, happy to chat about all this and learn from you ;)