Language packs are actually available for each version, but they’re not available on AMO, and pretty hidden on FTP.
Example, for 58 Beta:
I don’t know if Developer Edition is available on FTP? I couldn’t find any folder at a first glance.
As for future improvement, right now the priority is to switch the localization infrastructure to Fluent, that will give us space and tools to improve and add features (runtime fallback to different locales, localization updates separated from code, multilocale builds). The latter might be the actual solution for this kind of issues.