https://gerrit.wikimedia.org/r/135312 / I0f82b146fbe948f917c1c5d29f7469644d797e80 was merged in core and introduces a schema change for [[mw:page content language]]. The feature is desired on Wikimedia projects for T11360 / T37489 hence the schema change needs to be applied there.
Context and consensus:
Feature currently targeted for mid July:
The feature has been tested on the test.wikipedia.org and works as expected from a user's perspective.