Issue #1856976 by andypost, Soul88, ACF, aspilicious, vijaycs85, dawehner: Convert language_count() to the state system.

Issue #2005778 by webflo, naxoc, penyaskito, andypost, vijaycs85, plopesc, maggo: Convert language_admin_overview_form() to a Controller.

Issue #1754246 by webflo, swentel, penyaskito, andypost, alexpott, YesCT, dagmar: Languages should be configuration entities.

Issue #1620010 by dawehner, ParisLiakos, plach, jibran: Move LANGUAGE constants to the Language class.

Issue #1711070 by kid_icarus, tim.plunkett, xjm, droplet, zendoodles and cam8001: Convert tests to use ::$modules property instead of parent::setUp($modules).