David J. Peterson has talked about it a couple of times. Sometimes he’s allowed to coach people on pronunciation and other times he’s not. Sometimes in the edit they will change their mind about what they want the translation of a line to be after filming or splice together different lines, so even though they had him go through the effort of making a conlang and the dialogue, they fuck it all up after the fact by not making sure it matches what the final product says.
Side note, I always thought it was funny that they had the Dothraki repeating “armor” with two tapped /r/ sounds after hearing someone with an accent that doesn’t pronounce /r/ there say it. They apparently had understanding of English writing despite not speaking it.
Pretty much. Phonotactics, important geographic terms that are likely to be reused in multiple place names, a little bit of word order rules - like whether it would be the “Black River” or the “River Black”, and so on. As long as you keep some consistency, you don’t need to get into deeper stuff like conjugation, pronoun systems, how clauses are structured, and so on. George R.R. Martin is actually pretty decent at it, despite not being that interested in languages. Looking at random words from his books, you can usually tell whether something is supposed to be Valyrian or Dothraki just based on the aesthetics and the fact that there are some clearly related words.