TYPO3 Core

When enabling allowLanguageSynchronization you'd probably want to disable existing inlineLocalizeSynchronize (handled in DataHandler) features, such as showPossibleLocalizationRecords, e.g.

$GLOBALS['TCA']['tt_content']['columns']['media']['config']['behaviour']['allowLanguageSynchronization'] = true;
unset($GLOBALS['TCA']['tt_content']['columns']['media']['config']['appearance']['showPossibleLocalizationRecords']);

The differences are

allowLanguageSynchronization¶

→ works on tt_content.media and all related child items

inlineLocalizeSynchronize (like showPossibleLocalizationRecords)¶

→ works on each @tt_content.media.<child-item> individually

Find examples at https://docs.typo3.org/m/typo3/reference-tca/11.5/en-us/ColumnsConfig/Type/Inline/Examples.html#file-abstraction-layer, focussing on the following TCA type=inline properties in the appearance section (documented at docs.typo3.org/m/typo3/reference-tca/11.5/en-us/ColumnsConfig/Type/Inline/Properties/Appearance.html):

...
'appearance' => [
  'showPossibleLocalizationRecords' => false,
  'showSynchronizationLink' => false,
  'showAllLocalizationLink' => false,
],
...

Setting the showPossibleLocalizationRecords to 0 for the media field does resolve my issue. Thx!