If you are a Thai major in a university and you have a native speaker there, would you not speak thai? :)
This different language and it's explaination made no sense at all. And if you are friends and grew up together, would you not be able to speak each other languages after a time?

I would have wished, they would have not the "murder" case in it, which was clumsy and unrealistic and concentrating more of the MLs opening up their feelings for each others more in detail. They could even save money by only have eight episodes. ?

The show was ok-ish, besides the fluffy moments and some character growth of the MLs, it did not have anything special or worth remembering. Again a show which did not need to exist and if you do two languages, just don't explain it at all and nobody would care so much about it.