Fix translation_fallback in langdefs.py
I had inadvertently broken it in commit 14f96a55. It needs to be a dict, not a set of tuples.
This is currently blocking translation-status.py: https://lists.gnu.org/archive/html/lilypond-devel/2020-08/msg00106.html. As a result, I’d like to merge this outside the countdown so that translators can do their job.