Use Mail.app in English? I'm trying to use Mail.app in English in Yosemite. The reason for this is that when the app is in another language, it uses that language's headers for replies and can be very distracting to the other reader. 

So the two possible solutions are a reply-header fix (like Outlook's "Use English for message headers on replies and forwards and forward notifications" option) or a Mail.app language fix. There supposedly is a way to fix a single-app's language, but they don't work. 

Here is what I've tried: 

defaults write -app Mail AppleLanguages -array en

defaults write -app Mail AppleLanguages -array en-US

Editing the file:
/Users/[username]/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist

and adding an "AppleLanguages" array and then as the first String value, adding "en", which is what the defaults command does.
To modify Mail to use another language than the default system language open Terminal and enter
defaults write com.apple.mail AppleLanguages ‘(“preferred_language”, “default_language”)’.
In your example: defaults write com.apple.mail AppleLanguages ‘(“en”, “fi”)’ (assuming your default language is finnish)