[BUGFIX] indexed_search plugin cannot be translated

The experimental extbase plugin of indexed_search is not
translatable on Pootle because it is still using locallang.xml
instead of a XLIFF translation file.

Change-Id: Ia3a45573737f8be0f802bfdbf5bd4f36add66b07
Resolves: #58796
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/30103
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers