“Ich bin ein Berliner!”
One of the most famous examples of a language blunder in history. Even if you don't speak German, you probably know that it means “I am a jelly doughnut.” What the poor man meant to say was “Ich bin Berliner.” (I am a Berliner.)
I've seen this kind of thing a lot—people sort of abusing their use of other languages. I'm here to say be careful. It's one thing if it's a fantasy language or an alien language that nobody actually speaks, but if you put in an Earth-based language that even one person can understand, you can just about count on someone that understands the language is going to find it and, if there's a mistake, they won't hesitate to point it out. I do it with German—often.
And it's not all about mistakes. You can use the language perfectly, but it doesn't matter if it's not actually set on Earth. I have a little secret to let you in on about 'The Dark Crystal'. When she calls to Fizzgig, it's not some made-up Gelfling language. It's Serb for (My translation might be slightly off, but it's the right sentiment) “Good dog, good dog.” Basically what happened, so far as I can tell, is they found the person on staff that spoke the most obscure language and had them translate it. The problem is that, while most people won't understand, there are people who do, and they can't resist telling.
I'm not trying to scare people off from using other languages, but you need to check up on it (This is in an Earth setting. I strongly recommend avoiding Earth languages in a non-Earth setting.). The best thing you can do is use a language that you speak fluently, with proper conversational grammar. However, sometimes your character just has to speak Slovak. It's an understandable desire. What you need to do then is get someone that does speak the language you're using and get them to look over your translation or, better yet, give them the English and let them do the translation for you.
If you're just using simple phrases or words (Hello, goodbye, good night, yes, no) an online translator will likely work, but anything more complex than that and it's best to get some outside help. Not only will you get that grammar and wording correct, but there are things in different cultures that simply don't translate properly. Look at German. “Alte Schachtel” literally means “old box”, but if you say it to a German woman, she's going to be pissed. It's the equivalent of calling someone an old hag in English. In Japanese, they have a word that means nothing (Sadly, my memory fails me as to what it is.) that's simply said before a meal—that's its one purpose, and it does absolutely nothing else. Those are the little quirks of language you can't just throw together with an internet translator or a bilingual dictionary.
That's all I have to say on the matter—it's a matter of protecting yourself from rabid readers. Wait. One more thing (And I've sadly seen this in a manuscript before.): for the love of all that is holy, don't sound out foreign words! Find the real spelling!