On https://appleid.apple.com/ I changed the preferred language to French at least 10 times now, but it always comes back to German. Not immediately thus I see I can save the French language preference, but then I receive the next mail in German, I check my account and language is set to German. I live in Switzerland and thus our languages are French German and Italian...
I have experienced inconsistencies at times in the language used in communication received from Apple. I am not sure there is a clear answer. While a single Apple Id may be used to authenticate separate systems (e.g. Developer, iTunes, MobileMe) may handle languages differently.
From my experience, the following appear to affect the language used in communication from Apple.
For example, when purchasing from iTunes on my iPhone (Spanish), I received a receipt in Spanish. But when ordering from my Mac (English) I received a receipt in English.
To isolate the problem it may be worth reviewing which systems you use with Apple and in which languages to see what may be causing the change. If you are near an Apple Store, with this information, they may be able to help you sort our what is causing the change. Or perhaps report it as a bug in there system.