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.
Error messages are even trickier.
Punctuation is one of those areas few people think about. There are pitfalls here, too. I was once at a user acceptance dog and pony show, and a gentleman who spent his day in the hurly-burly of Wall Street approached me. I stared at him blankly, and quickly asked for more information. It turned out that an exclamation point at the end of some error messages made him feel like he was being castigated, and he didn't like it, particularly since it followed the instructions designed to tell him how to get better search results.