How much is too much? What do you feel is the best way to handle this?
I would use foreign words as an emphasis but only if it’s a common word that English readers would understand.
Swear words, names of things, or occasional things. Make sure you define it, but do so without it being clearly an aside to the audience. Like, “Shzbat, or [meaning],” is apparently quite jarring. I say this because a book I’m writing uses a number of made up words and the betas did not like how I opted to define them.
From experiences I’ve had, the following rules hold true:
If utilizing a proper noun: Generally no explanation is needed, unless the definition of the words in the proper noun are important to the meaning. For example, it would be: They went to [Name of Place], [translation of Place’s name into English]
If inserting a word or short sentence here and there for a necessary effect: Explanation not necessary if the context clues allow your reader to figure out what’s being said (utilize your beta readers). Otherwise, find a way to include the definition in a way that doesn’t take away from the scene.
If inserting any more than a sentence: It needs to be extremely necessary. You will need to again find a creative way to define the text for the reader. For example, if you have a fantasy story where John is speaking to Rick in a foreign language, but Michael (who does not know the language) is there listening in, you Michael could ask one of the two what is being said. They’ll likely summarize the text. Or in the case of John reading something written in the foreign language to others who don’t know the language, you’d want him to explain after he’s done reading.
Long story short, it depends on the scene/circumstance and how necessary it is to use foreign text.
I think the others have pretty much covered it. Unless you have two characters having an indecipherable conversation in another language.
Keep in mind that a lot of foreign words have been adopted as cliches in English, especially words from Western European languages (like saying that something is “magnifique!” is a really camp way of saying “awesome”).