Most of the time there is a dialect, the fact isn't used in the story. It just exists for the sake of diversity. Or the stereotypes it represents are so subtle you have to be Japanese to understand. Naturally, if the dialect is mentioned in the story, you have to explain it, and a tl note works for that. But I was rhetorically asking about the point in a general case.

________________
Active translations list
Completed translations list
Dropped translations

omg this is perfect. I would love if this were the standard.

someone below mentioned that different Japanese dialects were a matter of grammar instead of accent sounds. In the U.S, that would be the difference between standard English and something like say, African American colloquial English and perhaps some variants of Southern English, which would be hard to incorporate unless the translators were also speakers of those dialects and could translate accordingly, or they were linguists.

________________
;D

I prefer if they use standard English with a note. When people try to write in a non-standard English accent it often is just awkward and annoying. English dialects come with certain stereotypes about the speaker, and that stereotype doesn't always match the stereotype that comes with the Japanese dialect. Also, the writer would need to be very familiar with the English dialect to pull it off correctly. And in the end, it doesn't matter if the reader isn't familiar with the English dialect.

It gets worse. If the person writing the translation doesn't have the necessary familiarity with the dialect, it can be pretty insulting. In some series, though, dialects can end up being plot points--so the 'standard English with a note' route may not be always workable either. Making up a dialect would work, as would just recruiting somebody who can do the necessary checks for the real dialect you opt to use as a substitute.

As somebody who's decently familiar with Japanese dialects? philip72's list looks to be a very good one. Southern US's actually a bit too laid back for Kansai-ben, the logic seems to literally have been 'Of course Southern has the same connotations everywhere.' (I am certain people in Southern England would not agree...)