TYPO3 Core

David, I modified your patch a little bit, however it was the correct place and correct bug analysis! Good catch & thanks!

Great. Thx for that :)

Patch set 1 for branch TYPO3_6-1 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/30472

Just wasted a half day with handling the complications without this function working. Thanks alot!

Looks like this bug reappears in 7.6 LTS.
The fix from 3e6e8ec89c00b04485a3c6050e0cfdd9548a213e is commented out in TYPO3\CMS\Backend\Controller\FormInlineAjaxController on line 139:

        // Set language of new child record to the language of the parent record:
        // @todo: To my understanding, the below case can't happen: With localizationMode select, lang overlays
        // @todo: of children are only created with the "synchronize" button that will trigger a different ajax action.
        // @todo: The edge case of new page overlay together with localized media field, this code won't kick in either.
        /**
        if ($parent['localizationMode'] === 'select' && MathUtility::canBeInterpretedAsInteger($parent['uid'])) {
            $parentRecord = $inlineRelatedRecordResolver->getRecord($parent['table'], $parent['uid']);
            $parentLanguageField = $GLOBALS['TCA'][$parent['table']]['ctrl']['languageField'];
            $childLanguageField = $GLOBALS['TCA'][$child['table']]['ctrl']['languageField'];
            if ($parentRecord[$parentLanguageField] > 0) {
                $record[$childLanguageField] = $parentRecord[$parentLanguageField];
            }
        }
         */

Markus, is it still an issue with current v7 or 8 branch? If yes, plase open a new ticket with steps to reproduce the issue? I tried to reproduce but I couldn't.