Unlike Tolkien, George R.R. Martin never created full-fledged languages for his books, but he did make up a bunch of words. I guess they didn't want to replace those with different words from other real-world languages.

One of the challenges for David Peterson was to 'retrofit' the languages he created to what Martin had already made up. He coined a bunch of words and phrases for the books, and those needed to make sense and be grammatically correct in the newly created languages.

The phrase "valar morghulis", meaning "all men must die" in High Valyrian is a nice example. Peterson made the -ar suffix indicate the collective number. This is expressed by 'all' in English, but in many languages this is a separate number from singular and plural. He also added a paucal number (meaning "a few"). vala (man), vali (men), valun (a few men), valar (all men).