Usually, programs are written and documented in English, and use English at execution time for interacting with users. This is true not only from within GNU, but also in a great deal of proprietary and free software. Using a common language is quite handy for communication between developers, maintainers and users from all countries. On the other hand, most people are less comfortable with English than with their own native language, and would rather be using their mother tongue for day to day's work, as far as possible. Many would simply love seeing their computer screen showing a lot less...