[BUGFIX] EXT:documentation does not support translated manuals

Some extensions come with a translated manual and it should be used
if a match with the user's preferred language is found.

Fixes: #52332
Releases: 6.2
Change-Id: I84a87840db99fe2704c90bce8dc02f5bc39db4c1
Reviewed-on: https://review.typo3.org/24109
Reviewed-by: Eric Chavaillaz
Tested-by: Eric Chavaillaz
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn