memes

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.