Language pack maintainers can now merge strings from another branch. At the AMOS > Stage page, there is a form to copy translated string from one branch to another.
This way, language pack maintainers can for example easily pick their recent work on 1.9 translation and copy it into 2.0. The merge tool takes care of syntax fixing in case of 1.9->2.0 merge so $a placeholders use the new syntax etc.