[BUGFIX] Indexing files does not store the language

While indexing externals (e.g. PDF files) the language of the page
being rendered is not stored with the indexed file content. Thus
the file is shown/not shown in the wrong situations.

See detailed description in bug-report regarding the "situations".

Change-Id: I69eb1e222eb186b052eba93eaa9d5a80dd1e87ab
Resolves: #46261
Releases: 4.7, 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/18899
Reviewed-by: Henrik Ziegenhain
Tested-by: Henrik Ziegenhain
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer