In the settings of your language.
As I wrote: AdminCP -> Languages & Phrases -> Langauge Manger -> Edit Settings
There the field "Language Code". A list of correct languages-codes can be found e.g. here:
http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
I attached 3 screenshots.
Usually the language-code is correct, after you import a language. At least for the official languages. I wrote it in the description, just to make it sure it's there, because my little hack needs this information.
Br
Joerg