[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 first one.

Furthermore the language field for sys_file_metadata is removed from
editor form as the language is set by GET parameter already.

Change-Id: I64c97c8080196599e85b95e09e7eb89701670fa4
Re...