[BUGFIX] Fix language of placeholder for relations

When a parent record (e.g. tt_content) has inline relations
(e.g. sys_file_reference)and these relations show a placeholder
for records related to them (e.g. sys_file and sys_file_metadata)
then we need to pass the context of the language so that
the placeholders are shown in the correct language.

We now do so by putting the value of the language field
of the parent record into the data set and update it
in case child records are also translatable.

This way when going down the list of children, we always
know the language id of the last applicable parent element,
even if one element in between was not translatable (e.g. sys_file)

If we know the language, we can pick the correct record uid
out of the list of possible related uids, instead of
just picking the last one.

Resolves: #75040
Releases: master, 8.7
Change-Id: Iff1b5350138d3043df470d2bdbdfebc83ec8cc5a
Reviewed-on: https://review.typo3.org/54722


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>