As part of the i18n work that we did when I was in Heidelberg we added audit trail and it was one of the first steps toward version control for the translators, which is what we need (in addition to finer grained permissions and roles for the translate server with some kind of notification function so authors and admins of a locale can be notified when changes are made). Regretfully funding ran out before we could proceed.
In the meantime (for work I am doing in Boston) I have been encouraged to make changes to the UI using acs-lang for site specific needs (it is hard to resist) and it is clearly a can of worms waiting to cause problems (I see Malte's innocent commit error as an example of one of those worms, change a single message key and you have forked).
I do like your subclassing suggestion Gustaf. In addition to having a diff ideally the per-instance differences would have a "submit this group of translations to locale admin for inclusion in the superclass (e.g. the gender neutral changes that belong in German proper).
In any case, it's obviously not an easy problem.
