I had the same problem. I think it must be caused by the fact that i bought a charger in Apple Store Tokyo, and then entered my email address on the warranty form. Ever since emails in Japanese started appearing. When I now enabled Shared Photo Streams, my friends got invitations from me in Japanese too.
I was looking everywhere for language settings, but they all seemed in order. On iCloud that is.
It turns out it was the language settings on my apple-ID, not my iCloud-ID that was wrong, after a purchase at Apple Store Tokyo. Who would have thought that these two IDs were related? Or maybe they should be, but then why are they not even more integrated? Duplicate language settings?
This solved the problem:
1. Log into appleid.apple.com
2. Set the language.
The real problem however, is that Apple changed my language after a visit to an Apple Store. Come on! I already had a Norwegian apple-ID, a Norwegian credit card, and a Norwegian address. What could possibly make you think that I suddenly spoke Japanese? Ever happened that a tourist visited an Apple Store? Companies needs to learn that customers can make their own decisions, and often do, or want to do it, if they are allowed to do so. Examples of features that was designed not considering this fact: The play button on a Mac always launching iTunes, even though I'm already playing music in Spotify (and never use iTunes for music). Or Spotifys obsession with reporting everything you listen to, turning off the Private Session after 6 hours, so you have to make that setting every time you start playing music.