I18N: Localise language names in GUI

This is only the coding part. After this the .pot file needs to be recreated, and the translators need to translate the language names. The option --from-code=iso-8859-1 for xgettext is necessary because of the name Bokmål. Otherwise you get

xgettext: Non-ASCII string at common/language.cpp:49.

It was either changing the input code page or change the name to Bokmaal. I first (chosen) option has no regression for the user.