Thinking again about this issue whilst going through the grades help files. It seems very confusing having help files for the 1.9 gradebook in the same folder as help files for the pre-1.9 gradebook.
From chatting with Eloy:
stronk7: well, they are two different things afaik:
1) cleanup of unused string/help files
2) branching the rest of languages
1 requires 2 i.e. we cannot perform the cleanup properly without branching, but branching langs has it risks too:
1) translators must LEARN to use and commit them properly (potential nightmare)
2) translation interface must support branches
3) lang download (both from web page and from within moodle must know about branches)
(that's the summary IMO) but, for sure, for any cleanup, we need branching
Helen: or as Koen says: Maybe we need to redo the trick from 1.6 over again: create a new folder in CVS: lang2, clean all packs and move them over, have a new download page for 2.x language packs
stronk7: yes, that's an option... but in the long term (2.3, 2.4...) we'll be in the same situation. 
but perhaps "lang2" is a good moment, and we could start with it branched
stronk7: perhaps one discussion in forums could help?
asking translators and so on (they are the "key" to see if branching is going to be fairly managed by them)
they'll need to have one Moodle 1.9 another Moodle 2.0, Moodle 2.1 and perform each translation in the corresponding onw (committing to corresponding branch).
Thinking a little bit more on this: there is a problem with the help files: deleting them or moving them around will break backward compatibility.
May be we need to redo the trick from 1.6 over again: create a new folder in CVS: lang2, clean all packs and move them over, have a new download page for 2.x language packs