Why do some writers add foreign phrases to novels, without giving translation?
Well, you clearly ARE missing the boat! Probably because the foreign phrases that they use need no translation, even for most low-brow types, and also, because they figure that it makes them look high-brow. However, your sample of what I assume is supposed to be French indicates that you aren’t even getting that much. I only find it annoying when they get the grammar or words wrong.
I’d hazard a guess that the phrases in question are so common that they don’t generally need translation. If you do need it, however, there’s a whole internet out there, complete with translator programs, that should give you the general gist of what’s being said. Instead of whining, try doing a little research; you might actually LEARN something.