[BUGFIX] Wrong extension name in Mail::getSubject()

sf_register is used instead of SfRegister. As result the
translations can not be found.

This patch changes the extension name used to fetch the
translations.

Found 2 other places where wrong name is used.

Resolves: #55006
Change-Id: I5849076dad0c178ebf796d7b5151714c5d4f6c7a
Reviewed-on: https://review.typo3.org/26827
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer