TYPO3 Core

When you have a CE (tt_content) with some images created in the not default language and you change the sys_language_uid the relation handling + BE forms break.

When you translate the CE again opening the BE form of the CE will create new sys_language_uid records so you will see multiple file references appearing. Saving the CE will give an error:

ERROR Error handler    
Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1486233164: Child record was not processed | RuntimeException thrown in file vendor/typo3/cms/typo3/sysext/core/Classes/DataHandling/Localization/DataMapProcessor.php in line 631.

Situation before sys_language_uid change:

 tt_content.sys_language_uid = 10
 sys_file_reference.uid = 1111
 sys_file_reference.sys_language_uid = 10
 sys_file_reference.l10n_parent = 0

After change of sys_language_uid:

 tt_content.sys_language_uid = 0
 sys_file_reference.uid = 1111
 sys_file_reference.sys_language_uid = 10
 sys_file_reference.l10n_parent = 0

Expected:

 tt_content.sys_language_uid = 0
 sys_file_reference.uid = 1111
 sys_file_reference.sys_language_uid = 0
 sys_file_reference.l10n_parent = 0

Note for others who run into this issue, after you changed the sys_language_uid to 0 and create translation you will see records appearing like:

 sys_file_reference.sys_language_uid = 10
 sys_file_reference.l10n_parent = 0

But this should be

 sys_file_reference.uid = 1111
 sys_file_reference.sys_language_uid = 0
 sys_file_reference.l10n_parent = 0
 sys_file_reference.sys_language_uid = 10
 sys_file_reference.l10n_parent = 1111

Due to the fact that sys_file_reference.sys_language_uid isn't correctly set to 0 for the CE in the default language the l10n_parent value for the translated CE's/file references is not correctly set.