[Gimp-docs] new content localisation

[Vitaly Lomov]

Hi,
Whoever creates new content, please don't bunch untranslated languages
under english. I don't want the users see english when the rest of the
manual is in russian. It's just as bad as putting russian translation
under lang="en" by mistake. I understand it may cause some validation
issues at first, but that can be fixed at the parent node by filtering
out untranslated languages.
Vitaly.