document i18n conventions in docs/
(from redmine: created on 2012-09-26, closed on 2012-11-17, blocks #911 (closed), precedes #666 (closed), precedes #663 (closed))
(from redmine: created on 2012-09-26, closed on 2012-11-17, blocks #911 (closed), precedes #666 (closed), precedes #663 (closed))