Add localization support for www/<lang>/usergroups.html.

• www/<lang>/share/sgml/templates.usergroups.xsl: customization layer for each language.

• www/<lang>/share/sgml/usergroups.xml: translation of www/share/sgml/usergroups.xml. The translation teams can simply copy it from the language-independent directory and keep the translated items only; items which are not in the translated file are automatically inserted from the original file on the fly (so old items in the translated file are harmful).