TYPO3 Core

Most easily reproduced with sys_category and sys_category_record_mm, but this generally is true for all tables that have MM relation tables.

1. Have a sys_category entry in L=0
2. Have a record (tx_news or so) in L=0 or L=-1 using this category
3. sys_category_record_mm has one entry for this relation -> still OK
4. translate the category to L=1
5. sys_category_record_mm has now the original entry (L=0category -> L=0 record) and a false one connecting the L=0 record with L=1 category, the false one has a higher sorting and thus gets prioritized in FE

I tried several combinations of TCA settings (l10n_mode, dontRemapTablesOnCopy, localizeReferencesAtParentLocalization) which did not change anything.

I could not find the root cause for this behaviour.

This does not inherently break the FE rendering or categorization, but most likely lead to the situation that wrong translation labels are echoed into the FE.

It seems that this behaviour was recently introduced (one of the latest 9.5.x releases), because we have several big multi-language v9 setups that were working fine and recently we got customer feedback about this.