AppleId, language always changes
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.


The language of the device that
initiated an interaction using an
Apple Id
The language of the
country for which you are registered 
The language in the preferred
language setting


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.