fix(browser locale): Standardize browser locale (#608) (#619)

* fix(browser locale): Standardize browser locale (#608)

- move I18n class to original package to avoid possible breaks if
someone has been using it (although it is not part of the API) as it is
used in many tests

covers [BPO-543](https://bonitasoft.atlassian.net/browse/BPO-543)