Here's how to stop downloading on Windows if you have already started:
1. Close your Diablo 3 downloader.
2. Find "agent.db" file under:
Windows 7 users: C:\ProgramData\
Battle.net\Agent\
Windows XP users: C:\Document and settings\All Users\Application Data\
Battle.net\Agent\
Please note this is a hidden folder. Or alternatively, you can do a file search for "agent.db".
3. Open agent.db file with text editing tools such as Notepad.
4. Do a search for "selected_locale", change it to the way it was. For example, to change it back to English, put: "selected_locale" : "enUS"5. Save your changes. Reopen Diablo 3 and have fun!