Editor with language restriction to non-default language can't edit records of tables with inline fields configured with l10n_mode => exclude
In one of our projects we have a table for locations. This table has a file relation field for images, which is configured with 'l10n_mode' => 'exclude'.
The default language is German. There is a folder in the page tree, which contains only russian locations (they aren't translations of a german record, but created directly in russian).
When an editor, who is restricted to only the russian language, tries to edit those records, he gets an error: "Sorry, you didn't have proper permissions to perform this change. ERROR: Language was not allowed.".
This likely happens, because the file relation in sys_file_reference is created with sys_language_uid = 0 when l10n_mode is set to exclude, resulting in an error, because the editor can't access the default language.
I tested this in the most recent versions of TYPO3 8, 9 and 10. They all behave identically.