Combines the LanguageTag::Create() and Translate::get() (or nget())
functionality.

Also add an ABI test for the Kit class in the LibreOfficeKit unit
test, and a test for the translation function.

Change-Id: I78c48a8bbb434d6f204869290822830288022a53
Reviewed-on: https://gerrit.libreoffice.org/64882
Tested-by: Jenkins
Reviewed-by: Tor Lillqvist <tml@collabora.com>