Move to Crowdin platform

Notes:
* There are more languages on Crowdin than in the language option menu
* The languages order seems strange
* English folder is missing in Crowdin download (easy fix)
* It might be possible to refactor pycurl dependency into urllib2
* Push new translations by creating a file named ```crowdin_api_key.txt``` in ```build/``` with as content the api key (about 20 digits)