Flux form translation behavior supports have been removed from TYPO3 7.6 LTS - read here what that means for you.
This is a special notice about the Flux form option translation (the Flux counterpart for langChildren/langDisable in traditional FlexForms):
The support for this option has been completely removed!
This means that anyone using <flux:form.option name="translation" value="separate" /> or similar configurations involving the translation option will experience that the behavior toggles are now ignored. The behavior is now only supported when EXT:compatibility6 is installed - and even then it may be incomplete due to other removed supports.
We are continuously monitoring the TYPO3 commit stream but missed the commit that removed this support - or we would have objected to the removal, stating Flux still uses it. The argument in the deprecation notice is that "this was mainly introduced for TemplaVoila and usually only used in this context" - so unfortunately the author of that commit was not available that the now removed options were the only way Flux has (currently) of affecting localisation behavior of form fields. We don't agree it was right to cut this out, but it is unlikely it will be brought back.
Essentially, it means that unless your site is capable of running with EXT:compatibility6 installed you cannot use this option. There is no alternative, unfortunately.
Uncertain. There is work being done on a companion extension which converts FlexForms to IRRE - implementing it would certainly allow the IRRE records themselves to be handled in terms of localisation, but the localisation supports for IRRE may not provide all the same behaviors that langChildren did previously. Support may be basic, only supporting ON/OFF (e.g. not supporting per-value localisation). Whether or not this FlexForm conversion utility becomes part of Flux or gets shipped on its own is still not decided.
Even if it does: it is unlikely that such a feature would fully support converting existing values which were saved with those behavior toggles enabled. As it appears now, the FlexForm converter will only be safe to use on freshly created sites and must remain permanently in place as long as the site lives.
We apologise for the inconvenience and hope you will be able to work around it either by disabling localisation, using EXT:compatibility6 - or worst case, converting the fields which must be translatable into standard TCA and SQL additions.
The FluidTYPO3 team