Joseph does not reveal himself to his brothers. In fact, he goes out of his way to make sure that the brothers think he is 100% Egyptian. First, having lived there for at least 10 years, his complexion is probably more like that of an Egyptian. There’s probably not much change there, just a darkening of skin color, but, culturally, Joseph is very Egyptian right now.
As the governor, he’s going to be in Egyptian clothing and likely adopted Egyptian mannerisms both to make it easier to do his job and because he’s been surrounded by it for so long. Included in those mannerisms is likely the adoption of makeup. Makeup was not limited to Egyptian women at that time, so Joseph would appear very different than his brothers remember, and they remember an annoying kid, not the governor of Egypt.
Adding on to the ruse, Joseph is using an interpreter to speak to his brothers. Yes, he fully understands what they’re saying to him, but they believe he only speaks Egyptian. Interestingly, this is really the first time an interpreter is mentioned in the Bible. It’s the first tangible evidence of a consequence to the Babel story told so long ago. With all of the other journeys to different places and different lands, there’s been no mention of any variation in language. Abraham even traveled to Egypt himself, but there was no mention of language at the time.
Does this mean that Babel didn’t have an effect until now? No. Not at all. In fact, languages were probably different from town to town. Probably not radically different, but it doesn’t take much to tell Brooklyn from Queens. This is is one of those things that is not mentioned until there’s a reason to mention it, something that the ancient Hebrews understood implicitly. Of course, people in different lands spoke different languages, why would it be anything different?
This story actually needs to mention it because readers would probably assume that Joseph would speak to his brothers without an interpreter. He obviously speaks their language, so why would he need one? The story needs to tell us that this is part of Joseph’s plan to remain unknown to his brothers.
Joseph has obviously invested a lot into this disguise, and it was necessary to go into this before we get into the dastardly, villainous, soap opera plot he will unleash upon his brothers.