TYPO3 Core

Things that the old implementation didn't take care of:
1.) Setting the language to -1 for all wasn't transferred to childs, as the check was > 0, it should be != 0 or just set it always.
2.) Bug #63092
3.) Related to bug #63092: If one changes the language and creates a new child record prior to saving it will be created with the old langauge uid.

Maybe not transferred to existing children but I think newly created children should have -1 if the parent has -1, shouldn't it?

In regards to existing childs I have commented further in that regard in ticket #63092

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/52061

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/52061

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/52061

Patch set 9 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/52061

Patch set 10 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/52061

Patch set 1 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/53206

Patch set 2 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/53206

Patch set 3 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/53206

Patch set 4 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/53206

Patch set 5 for branch TYPO3_7-6 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/53206

The backport to v7 has been merged (https://review.typo3.org/#/c/53206/)
Few notes:
1) Now NEW irre elements (including FAL relations) will get a sys_language_uid of their parent.
2) There is no automatic migration of the old record having inccorrect value in the sys_language_uid.
3) The issue has been solved on the FormEngine level, thus you need to make sure you have
sys_language_uid field in the showitem list in your TCA.

Often you don't want to show the sys_language field to the user, so there is a little trick, which will make stuff work without showing the field to the user.
You just need to create a hidden palette with sys_language_uid in it.
e.g.

'showitem' = '... --palette--;;hiddenLanguagePalette,';
'hiddenLanguagePalette' => [
    'showitem' => 'sys_language_uid, l10n_parent',
         'isHiddenPalette' => true,
],