In older Mac OS X, it is possible to set the format language in the system preferences. However, in macOS Sierra, there is no such an option any more.
From this post, we can use Terminal.cpp to force it. To force the format language to English (Australia), simply type
defaults write NSGlobalDomain AppleLocale en_AU
To do that, just use ‘defaults’ command.
The usages is simply:
default [command] [domain] [attribute] [value]
If I want to change the default language of com.apple.mail to be English while the whole system is in Chinese, just type in terminal:
defaults write com.apple.mail AppleLanguages ‘(“en”, “zh”)’
It will search for English language pack first, then Chinese.