For people who speak or understand thai language, why there are subtitles on those dialogues between Sorn and the girl? Weren't they speaking thai in these series? 

Yes,that's what I was thinking too. It could be a dialect which is spoken in a typical region of Thailand.

They are speaking in the Isaan dialect, which is Lao spoken in the northern part of Thailand, which most mainland Thai speakers can't understand.

 vavim:

They are speaking in the Isaan dialect, which is Lao spoken in the northern part of Thailand, which most mainland Thai speakers can't understand.

No, they are not speaking Isaan. Isaan is the most prominent northEASTern dialect of thai. What they speak is Kham Muang, the actual northern dialect of thai. Isaan and Kham Muang has a lot of similarities but are quite different. I can confirm.

 vavim:

Oh, what. I asked my Isaan Thai friend if this was Lao and she confirmed. But she doesn't speak the greatest Thai since she has grown up in Europe so she might've just messed up lol

Lol, yeah, I think I can understand why. Isaan and Kham Muang has many similarities on some basic vocabs and accents. I am familiar with Kham Muang so I can tell. I can also understand Isaan dialect to certain extent because of their similarities, especially in a more centralized/tamed version that they often show in media.