Will my interface or game language change if I switch regions?
Your game language and interface will remain the same when you change regions. If you purchased the full-language version of Diablo III, you will be able to play in all available languages in all regions. To play in a different language, you will first need to download the Diablo III game client for that language from Battle.net.
If you purchased the Korean, Russian-language-only, traditional-Chinese-language-only, or another language-limited version of Diablo III, you will only be able to switch to the languages you’ve purchased. You will, however, be able to play in all three game regions using the language you purchased. Note that the Korean-language client does not contain the real-money auction house inferface.