I18N: Add support for language names translation

The option --from-code=iso-8859-1 for xgettext is necessary because of the
name Bokm\xE5l. 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.
The first (chosen) option has no regression for the user.
I18N: Update Dutch translation