You ask a very good question. One's language can be colloquial, formal, or very formal, depending on the circumstances. A good analogy is the way one dresses. One can dress in jeans and a tee shirt, a shirt with a collar and khakis, a suit, or a tuxedo. One's dress should be appropriate for the occasion. And the various garments should go together. One would usually not wear a blue proletariat work shirt, jeans and a bow tie. Also not a tuxedo with shabby sneakers. Words such as "interlocutor" should be reserved for very formal language. Look at this: I was in a bar the other night and met a great guy. We talked about all kinds of stuff. My fellow interlocutor was very smart.
That doesn't sound right.
Now this: Ladies and gentleman, I am very honored by this prestigious award. I will cherish it to the end of my days. It is really good shit.
That doesn't sound right either.
Student or Learner