I think I have the solution to this issue.
What Happened to me:
When I send invitations to my friends, they received the invitations in Japanese (I was in Japan for 2 years, that's when I created the MobileMe account, perhaps it is related).
I changed the language of iCloud (in User Name -> Preferences, after signing in into http://www.icloud.com). The language was already "English".
But the problem persisted:
Go to: https://appleid.apple.com, (Manage Your Account, once signed in, go to "Language & Preferences")
There, you can type in the language for your inviations (English). Check the primary and the alternate mail addresses just in case. Sometimes, if the primary mail address is not iCloud or me.com, the invitations are not sent propperly.
After that, you can save your settings. The invitations are sent in English.
Problem solved on my side.