The key part is the SwUserFieldType::GetValue() hunk, the field type has
to always use the same locale, which means if we get an SwCalc reference
that works with the document or field locale that has to be switched
temporarily.

Change-Id: I26ff18e74f477729a66b066c4baf6d215a7685bc
Reviewed-on: https://gerrit.libreoffice.org/58492Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins