Fix for loading country specific languages codes

[lyralemos]

With this fix, country specific language codes are correctly loaded. Fixes plone/Products.CMFPlone#949

As language codes are obtained via lang property in the html tag, there is an inconsistency with Plone locales code base. For instance, Brazilian Portuguese lang attribute is pt-br, but in plone locales is referred to as pt_BR.

This code does a simple transformation on the language code so that correct translations are loaded.

[davilima6]

@lyralemos will sign and send over the Contributor Agreement today. Thanks! (also kudos to @claytonc who initially helped with the solution=)

[agitator]

lgtm

[lyralemos]

I've already signed the agreement.
Code was committed using a different e-mail which I just added to my GitHub account.

[davilima6]

I'd squash all commits before merge.

[thet]

LGTM

[gforcada]

@lyralemos @thet when this got merged Jenkins started failing on the JavaScript job: https://jenkins.plone.org/job/plone-5.1-js/ any idea why the tinymce tests fail?

[lyralemos]

I've managed to run the tests locally and realized were the problem is.
For some reason tests are falling if I don't check if currentLanguage is different that en-us.

Should I create a new PR? Or can I still commit changes here?

[gforcada]

@lyralemos a new pull request would be best, thanks for having a quick look at it!!