There is the LocalisationUpdate extension  but only few people use it. It is slow and needs a special configuration with cron. If we could integrate it into core, make it fast enough so that cron would not be needed it would allow a lot of third parties to enjoy the blazingly fast localisation updates (under 36 hours) that Wikimedia projects currently have. To make it fast enough, it is likely that a separate service needs to be implemented. It could be standalone or part of some MediaWiki instance. It should be secure and allow querying only needed data. Ideally, wikis would no longer need git to update their l10n.
Proposed by the Language team at https://www.mediawiki.org/w/index.php?title=Outreach_programs/Possible_projects&oldid=902079#Generic.2C_efficient_Localisation_Update_service , see also https://www.mediawiki.org/wiki/LUv2
Since MediaWiki core is affected, we need the buy-in from the maintainers before proposing it as a GSOC / OPW project.