My roommate is a graduate student and Spanish GA at University of California, Berkeley. As he was grading Spanish essays the other day we got into a conversation about language. One of the main points we came to was that when you use a foreign language you can’t use it as you would your own. Meaning, colloquialisms, phrases, metaphors and manners of speech don’t always translate directly, or at all. His classic example is a student who likes to say cabrón casually, the same way he would say fuck casually. Trouble is, the Spanish don’t do that. The closest thing to what he wants to say is cabrón de la chingada, which he explained to be like “what’s with this fucking thing.” Even that would only be used with friends. Simply saying cabrón is like saying fuck your mother, and while we Americans might be able to say that jokingly, expect to lose a few teeth if you say that to a Spanish speaker.
Similarly, there is a lot of the Japanese culture in the language. Insults and profanities don’t function the same way because it’s more about the situation of how you disrespect the person rather than the actual word.The politeness factor of the culture factors in heavily here as well. The most insulting thing you can say to a person in Japanese is a word that means “you are so worthless you shouldn’t even exist.”
When you speak the language you want to try to put yourself into the mindset of a Japanese person. Though it may be difficult, this will help to be better understood. If you don’t, expect some misunderstandings and some difficulty relating to others. There’s a good deal of leeway being a foreigner using the language, but you can’t always rely on that. The best advice to have is imitate native speakers around you and use your best judgment.