If the fallback language tag based on available localisations would
have the same language as the original Bcp47 tag string, use a tag
based on the original.

For example, if the user locale is 'de-CH', the localisation for plain
'de' will be used automatically anyway, without making the LOK
language tag 'de' which would actually end up being stored as 'de-DE'
and thus be misleading.

We will need to know that it's the 'de-CH' locale we are actually
working with when handling Swiss German translations by replacing 'ß'
characters in a generic German translation with 'ss'.

Do it only when comphelper::LibreOfficeKit::isActive() to avoid
modifying behaviour in the "normal" cases.

Change-Id: Ib5bdfe07fd53251b42122389cb81782ef87b65c5
Reviewed-on: https://gerrit.libreoffice.org/63446
Tested-by: Jenkins
Reviewed-by: Tor Lillqvist <tml@collabora.com>