TYPO3 Core

https://docs.typo3.org/typo3cms/extensions/form/Concepts/FrontendRendering/Index.html#translate-form-definition

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Due to compatibility issues, the setting translationFile is not defined as an array in the default configuration. To load your own translation files, you should define an array containing 'EXT:form/Resources/Private/Language/locallang.xlf' as first entry (key 10) followed by your own file (key 20) as displayed in the example above.
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• If anyone override translationFile with a single string they don't want to have a default value there and overrides the whole behaviour
• If anyone override/changed translationFile with numeric keys they will have the default file as 10 or doesn't use them, so they don't care about core changes here.

If I have to carry the entry for the default file every time I want to override these setting that would be much unnecessary code. Additional the default file(e.g. the location/path) can never be changed in core without be a breaking for any extension that overrides just a single label. We should check to change the default config syntax to a array now and do that in 10 if anything will break up.

The current translation file configuration within the current ext:form core setup is a string.
If we change the translation file to an array, we have to define the ext:form core tranlation file within some array key, lets say "10".
In the meentime, an integrator has defined some additional translation files and therefore the integrator has switched to the array notation within its ext:form setup override.
Lets say the integrator has choosen the array key 0 for the ext:form core translation file and 1 for its own.
If we ship a new ext:form core configuration with TYPO3 8 / 9 with the array notation, we change the behavior of the integrators TYPO3 instance.
In this scenario the array key 0 contains the ext:form core file, key 1 the integrators custom file and within key 10 we have the ext:form core file again.
I will try to deprecate the string syntax within TYPO3 v10, this could make the change in version 11 available.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 6 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 7 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 8 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 9 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593

Patch set 10 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/60593