Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #365615 followup by attiks, YesCT, Gábor Hojtsy, mgifford, Albert Volkman, plach, webwarrior, David_Rothstein, penyaskito, smokris | yang_yi_cn: Fixed Followups: Language detection not working correctly for most Chinese readers (and add a user interface for all browser language mappings).

Issue #2347465 by tim.plunkett, dawehner, cilefen: Convert all instances of #type link/links to convert to use routes.

Issue #2329847 by tuutti, rachel_norfolk, Cottser | davidhernandez: Use Attribute to replace hard coded classes in language-negotiation-configure-form.html.twig.

Issue #1898422 by joelpittet, lokapujya, Salah Messaoud, stevector, Les Lim, pplantinga, jenlampton, drupalninja99, tlattimore, johannez | c4rl: Language.module - Convert theme_ functions to Twig.

Issue #1987726 by vijaycs85, disasm, YesCT, pdrake, tstoeckler, dawehner, Xano, likin, stella, andypost, prateek479, Gábor Hojtsy, kgoel: Convert language content page related callback to new style controller.

Issue #2254495 by ParisLiakos, alexpott: Move language_admin_predefined_list to ConfigurableLanguageManager.

Issue #2166915 by ianthomas_uk, sun | plach: Remove uses of deprecated language functions in tests and procedural code.

Issue #2121175 by Codenator, tim.plunkett, cosmicdreams, jayeshanandani, grom358, ianthomas_uk, Xano: Remove usage of drupal_get_form().

Issue #2171071 by jessebeach, dawehner, chakrapani, longwave, Gábor Hojtsy, xjm, Wim Leers: Rename drupal_add_library() to _drupal_add_library() and remove its uses.

Issue #2187735 by ianthomas_uk, Sutharsan, andypost: Add removal information to docblock of all @deprecated functions.

Issue #2196901 by divesh.kumar, longwave: Convert all calls & docs references to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in language module.

Issue #2165155 by Berdir, tim.plunkett: Change $entity_type to $entity_type_id and $entity_info to $entity_type/$entity_types.

Issue #2086485 by brianV, jibran, pwolanin, disasm: Convert language_negotiation_configure_form() to the new form interface.

Issue #2005716 by tim.plunkett, msonnabaum, dawehner, alexpott, effulgentsia: Promote EntityType to a domain object.

Issue #732022 by vijaycs85, Wim Leers, dawehner, nod_, kscheirer: Drupal_add_tabledrag() still using drupal_add_(js|library)(), should return array for #attached.

Issue #2045931 by PerthSam152, LinL, adsw12, joelpittet: Replace all module_exists() deprecated function calls.

This reverts commit cc84f390 - bad
commit message.

Issue #2089635 by tim.plunkett, disasm, larowlan: Convert non-test non-form page callbacks to routes and controllers.

Issue #2003592 by andypost, mhagedon, webflo: Convert language_admin_add_form() and language_admin_edit_form() to a Controllers.

Issue #2080429 by mrsinguyen: Remove Unused local variable  from /core/modules/language/language.admin.inc.

Issue #1946426 by ramlev, pfrenssen, foopang, webflo, David Hernández, vijaycs85, disasm, tim.plunkett: Convert all of confirm_form() in language.admin.inc to the new form interface.

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

Issue #2031197 by webflo, kfritsche, penyaskito: Language configuration entities should be created in English at all times.

Issue #1833022 by e0ipso, plach, YesCT, kfritsche, penyaskito, vijaycs85, nod_, cam8001: Only display interface language detection options to customize more granularity.

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

Issue #1668866 by ParisLiakos, aspilicious, tim.plunkett, pdrake, g.oechsler, dawehner, Berdir, corvus_ch, damiankloip, disasm, marcingy, neclimdul: Replace drupal_goto() with RedirectResponse.

Issue #1845034 by RobLoach, sandykadam, nick_schuch, cam8001, Gábor Hojtsy: Convert standard.inc to Drupal\Core\Language.

Issue #1987066 by Kevin Morse, JohnAlbin, rteijeiro, echoz, ry5n: Rename files to match CSS file naming convention.

This reverts commit c4e52bb6.