You should a a noun after. I met a Japanese person or I met a Brazilian person. The sentences could be interpreted as an insult or just incorrect without it. Nationalities can also be used as adjectives in English- that is why you should clarify with a noun. “I met a British” would be incorrect.

Either is fine, in all cases. You can choose to tell your listener what gender they are, or you could choose to not tell them. It's probably more polite, and more common to say, but grammatically you can choose either. I was going to say "I don't know what made you think that there are some nationalities you can't do that with, that's not true. It's perfectly fine to say I met a Japanese at my class." as that is what I would say. But - according to https://www.ef.co.nz/english-resources/english-grammar/nationalities/ that's not true, and you should say a Japanese person. There are a few other exceptions there too.

Yes, you can say, "I met a Brazilian in my language class today." (Use "in" class.) You are correct. Saying, "I met a Japanese in my language class today," would be very strange. Here's a full list: http://www.ef.com/english-resources/english-grammar/nationalities/