Having only a HEAD version for language packs causes major problems and inconsistencies

The language packs other than the en_utf8 one have only a HEAD version (i.e. no STABLE branch) since 1.6. This causes major problems.

1) Some modules breaks in 1.6.x with the introduction of new strings. See bug MDL-6644.

2) Deleting legacy strings like $string['studentviewoff'] creates regressions in the interface, when using up-to-date language pack. "Some strings that were translated are now in english. What's happened?"

3) Not adding new strings and not removing legacy ones (for conserving backward compatibility issue of point 2 above) doesn't allow smooth development, nor correct testing on development version using other language packs.

I suggest that we think about a mecanism allowing to have a stable version of language packs along with a dev one, so that could be resolved elegantly.