Multilanguage flows

Currently, it is possible to handle a bot flow in a single language. This requires to design multiple bots for multiple languages, that is time consuming and error-prone. It would be great introducing the possibility to handle multiple languages within the same flow. Here a possible idea: - setting in the flow a well-known attribute that represents the current language (that may be determined by different mechanisms: passed by the web page / selected by the user / automatically detected from its first question). - specifying replies and labels (including the forms label and error messages) in different languages for the same action; - specifying the NLP training phrases in different languages for the same action. A specific syntax in the prompt definition may be enough, for instance: ``` [#en]You inserted a wrong value.[#it]Il valore inserito non è valido[#fr]Vous avez inséré une mauvaise valeur. ```