They made a language off of a video game?
How did High Valyrian become an offcial language after it was made up by Game of Thrones?
High Valyrian is a constructed language by linguist David J. Peterson and George R. R. Martin, used in the seriesof books "A song of Ice and Fire" and in the HBO television series "Game of Thrones". One of the main characters Daenerys Targaryen, speaks High Valyrian, and the language soon became very popular, David J. Peterson colaborated with 3 of his students to create the Duolingo course, and as you can see, there's more people learning High Valyrian than Czech, so that means that, indeed, the language is very very popular.
High Valyrian is not just a game, but a TV show. The language is mostly used in the TV Show. High Valyrian not an official language of any kind.
People have fun learning it, they can understand High Valyrian phrases on the TV show without looking at subtitles, they can even communicate with other High Valyrian speakers...
Of course learning English, Chinese or Spanish makes people able to communicate with more people in the world, but why not learn a fictional language? If you're into that fandom, you may like to feel "immersed" by understanding that language; if nothing else, it's good training for your brain cells, because you have to memorise words you can't derive from any language you know.
The fact that there's people wanting to learn High Valyrian (or Klingon, or Quenya / Sindarin) is the reason for the existence of websites teaching those languages.