The problem was that in case the shape text has an explicit language
set, then the shape language will have no effect on shape text language;
need to clear those attributes explicitly.

The const_cast<> for the EditTextObject should be OK, since we change
the shape's item set anyway, so we don't omit any needed notifications.

Change-Id: I721e3f0aefa103a6bfa320a671da2ee1e039fa38
Reviewed-on: https://gerrit.libreoffice.org/72904Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins