Invoking GNOME applications with LANUAGE=C application in a terminal seems to work fine (I've not yet experienced difficulties), I assume it is a specified (GNOME) behavior. What would be the equivalent way to controll language of KDE applications with a shell variable (not necessarily invoked in a KDE desktop, e.g. in Unity)?
The command
LANGUAGE=C application
is wrong. It may seem to 'work', but it's a coincidence.
LANGUAGE=X application
'works' too in that sense.
The LANGUAGE variable expects a colon separated list of language codes. Since neither C nor X is a valid language code, GNU compatible applications fall back to the original language, i.e. English.
LANGUAGE=en application
would be correct if LANGUAGE in the session environment is set to some other language, and you want to start a particular application in English.
To start non-GNU compatible applications in English you can use
LANG=en_US.UTF-8 application
