Some interesting comment from Update Center bug 1802. It indicates how Update Center's notifier works in term of language.
The notifier can be started in several ways: 1) At login (inherits login locale) 2) By updatetool (it is started in the background a few minutes after updatetool is launched) (inherits updatetool's locale) 3) Manually via updatetool --notifier (inherits the shell's locale) You can use updatetool -n --shutdown to stop all running instances of the notifier.