Hi- I am confused too,but I suppose I haven't read the instructions If we make changes, do we have to make them to 2.0 AND 2.1?
Yes, you have. And to 2.2dev too, because sometimes it will be the main version.
If you translate new (untransated) strings, you can translate them in one branch and then (after mantainer commit your translation) use megre tool. If you change something in translated string, you should change all branches manually.
Please note that only language pack maintainers are supposed to deal with branches. Contributors can just select one version in the translator (like 2.1 only) and translate that. After they submit the work to the language pack maintainers, the maintainers are supposed to apply the contribution to all desired branches.
Of course, maintainers should have helper tools to ease their job. See http://lang.moodle.org/mod/forum/discuss.php?d=1005 where a solution for simpler propagation of changes is discussed.