bugfix for wfBCP47 and code coverage

bugfix for wfBCP47 and code coverage

Language code are case insensitive. The BCP 47 recommands nice
formatting nonetheless. This patch enhance our formatting:
- tags preceded by the private tag 'x' are now lower case
- 4 letters tags are now lower case with first letter uper cased

Please note the RFC seems to have a bug for az-Arab-x-AZE-derbend
which should be az-Arab-x-aze-derbend .  I have changed our test
to reflect this and added a comment for later reference.