Often when we read a story, we want it to be authentic. That's a big reason as to why people mention other languages in their story. After all, why else would there be a French guy? We need someone to show off our ability to swear in other languages.
That being said, if you don't know the damn language or a native speaker who can help you write the phrase in a given language correctly, don't use that language. It's very simple. There are people from all over the world on Wattpad. If you don't want someone to feel insulted because you said that their mother is like a drive-thru with more nefarious purposes instead of the complement you were trying to describe, make sure you're completely correct. And Google translate is not good enough to check crappy translations (because it's only good for translations of single words, not phrases. Especially not colloquial ones).
Assuming you know that that your translation of "My God, your dog is one fat sausage! Can we eat him?" is correct, there's the problem of making your text look sort of natural in context. This is one that people fail at often.
Speaking as someone who does rely on their other languages to be understood, it's not like people throw out phrases willy-nilly. The only time people ever try using another language to be understood is when they honestly don't know how to say it in another tongue. For instance, if someone was asking me something complicated in Swedish -- a language I have a super limited vocabulary in, by the way -- I would start jabbering in every other language I know in a sad attempt at finding a similar word. This is true because I have honestly done that (that poor pizza guy. His ears will forever be scarred).
What I would never do is start talking in a language that no one else knows for no reason (unless I'm flat-out drunk. In which case, who doesn't? English is not a good drinking language). So stop making characters in books do so (or continue to do so if they are in fact flat-out drunk).
Foreigners/polyglots are not weird people. We don't randomly start exclaiming stuff in some other language if we can't help it. But it seems that writers are adamant that we do so. Vad fan.