There is a "banana checker", which checks that i18n json files are valid and that there is proper correspondence between en.json and qqq.json.
It is enabled for some extensions now. It should be configured for all MediaWiki extensions. Ideally, it must also be voting, although initially some extensions will have to complete their qqq documentation first, but it should be be voting for those that already have complete qqq.
Patches to review: https://gerrit.wikimedia.org/r/#/q/status:open+topic:banana,n,z
As of Feb 20th 2017:
At the moment all extensions of mediawiki/extensions.git with valid qqq.json have a banana checker running and passed.
The following extensions does not have a banana due to problems with missing message keys in qqq.json or missing qqq.json at all:
skins: -none- extensions: -none-