fdo#82757 call convertStringToNumber and detectNumberFormat with same locale

So that they agree (among others) on date recognition patterns.

Change-Id: I964142702b5bb23bca2c62433e52f76dfa4bdacb
Reviewed-on: https://gerrit.libreoffice.org/11003
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>