Hi Ernesto,
there is no solution.
Unfortunately from incomprehensible reasons (hello developers!) language of Workflow Designer app is depending not on system (Windows) display language but it is depending on Regional format (data = meant that format of date, time, currency, numbers, decimal symbols etc.).
Problem with this "workaround" is that if you change Regional data format (because you want to change language for Workflow Designer app) it will affects all other application which using system Regional data format - e.g. MSOffice apps (Outlook, Excel, ... etc.) and it can breaks their function.