It's our first and most powerful tool, and separates us from the armadillos of the world. Language enables us to make connections, build complexities, and to influence each other. No one manages the language from a global level, but a lot of people decide what the language will be for specific parts of the site. This is problematic on multiple levels. In other cases, she is simply the watchdog, enforcing and normalizing the language decisions made by a group of people.
Language is a critical part of the site metaphor, the user interface, and the user experience.
The typical Language Czar will debate synonyms, review word choice, determine punctuation, and establish requirements. She'll build vocabularies, write standards, develop requirements, and review the implementation. She will also shine a spotlight on some key areas that are often cursorily handled. Guidance text is a tricky thing. Too much of it can mean that you think the user will be unable to use the site due to his idiocy or your incompetence in designing and building.