The new version of MediaWiki will be on test wikis and MediaWiki.org from 18 July. It will be on non-Wikipedia wikis and some Wikipedias from 19 July. It will be on all wikis from 20 July (calendar).
MediaWiki system messages will now look for available local fallbacks, instead of always using the default fallback defined by software. This means wikis no longer need to override each language on the fallback chain separately. For example, English Wikipedia doesn't have to create en-ca and en-gb subpages with a transclusion of the base pages anymore. This makes it easier to maintain local overrides.
The action=growthsetmentorstatus API will be deprecated with the new MediaWiki version. Bots or scripts calling that API should use the action=growthmanagementorlist API now.