What do you hear at the café? What do you hear during a lecture? What do you hear when you are with your good ol’ grandma?
One of the tricks to writing believable dialogue is to consider not only who is speaking (more on this here), but also where they are and who is around them.
Your character might want to avoid cussing at teachers during a verbal exam (well, at least, not cuss them out loud), but their friends could find it odd the MC hasn’t addressed in a rather colorful way the guy who just bought the MC’s love interest a drink.