It mostly comes down to this, which shows up contained in either one or two sentences:
most characters should take the appropriate one as a specialty. Educated or well-traveled speakers will have tried hard to rid themselves of their dialect, and may have standard specialties (see ArM5, page 66).
This has shown up repeatedly in books since GotF. However, that sentence is completely absent in AtD.
Meanwhile, the penalty is written as:
Same language, different dialects -1
That shows up in most the books, too, usually in a table.
I would propose fixes of:
most characters should take the appropriate one as a specialty. Educated or well-traveled speakers may have standard specialties (see ArM5, page 66).
which allows other choices for them but doesn't specify that they're trying to "lose" a "dialect" and
Same language, no shared dialect -1
which handles the lack of dialect for the reader and others. As for the locations, ugh:
GotF p.138
TSE p.41-42
AtD p.10 (lacks the first sentence(s))
TL&TL p.143
TCI p.21 (lacks the second in a table but has a note about it)
F&F p.19-20
TC&TC p.185 (lacks the second in a table but has a note about it)
LotN p.10-11
BS&S p.18-19
I'm not sure if it shows up casually in other spots that have some notes about languages such as this thing in Ancient Magic p.94. There "dialect" is used where they would be noted as similar languages rather than different dialects elsewhere, as the penalty is -2 rather than -1. That may need its own fix. I didn't spot it in HoH:MC, RM, nor HMRE, all of which contain additional languages.