i18n: update translation files and set up a Transifex project

Sonata is now translatable at:
https://www.transifex.com/projects/p/sonata/
i18n: fix broken translation files so they can be uploaded to Transiflex
i18N: add a README information about translation
i18n: now use Transifex as a translation plateform

Merge branch 'i18n-transifex'