Help:Extension:Translate/Unstructured element translation

Things to cover:
 * Applies to ui messages like sidebar
 * Can be used with
 * with page content
 * Better for certain kind of pages
 * How to setup a group for this
 * How to add messages to the group

also describe the "twn main page" method ( trick; try to steal some documentation from Commons; supposedly documented in commons:Commons:Localization and m:Help:System_message (can't copy anyway, not PD).