It goes by either file extension or the hashbang line. But I think it only make the determination when you open the file; it doesn't change the language when yo

It goes by either file extension or the hashbang line. But I think it only make the determination when you open the file; it doesn't change the language when you edit the text. You can manually set the language pretty easily though via the command palette or by clicking the language in the bottom-right.