Language Switcher

Mac
Language Switcher adds key "AppleLanguage" to plist in ~/Library/Preferences/(applications plist).plist file.

If you add only "English" for Mail.app, it won't let users choose other encodings than UTF-8 and English encodings. If you want to use other encodings, you can _add_ language, for example "Japanese", to that key. (By keeping "English" first in the list, Mail.app UI will stay in English.)