Sprout Email Plugin and Multilanguage. How to translate SubjectLine?
First, we'll be adding native support for localization in Craft 3. This is a bit trickier in Craft 2 right now.

Two thoughts. You could set up 3 different emails (each managed in a different language) to be triggered off the same event. Not ideal, but an option. 

Aside from that, Sprout Email does run the subjectLine field through renderObjectTemplate so you could use a field in whatever is triggering your event as a dynamic variable for the Subject Line.