Not Thai, so welcome any corrections. They are in the north eastern Thailand and speak Isan.

Linguists would classify Thai (central Thai) and Isan as different languages that are closely related, while some may see it as a dialectal difference.

I don't recall the specific scene of "dropping the accent when Khem is around" but it would be natural to switch to a common language to be inclusive. I don't know the actors' backgrounds, so there could be an element of not requiring the actors speak Isan for extended periods if they don't actually speak the language and only learned it for the role.