IMO they should, speaking english really helps with marketability.
most fighters do anyway, for example :Anderson tries his best(doesnt talk with a translator next to him) even though hes bad at it;he is still trying to connect with his non-portugese speaking fans.
Machida's english is also getting better and better.
Benson Henderson vs Anthony Pettis 1 Highlight vid (270k views): /watch?v=MhTD2NhrdYY
Simon Marcus vs Joe Schilling III (Glory LMS) Highlight vid: /watch?v=5nAEJKikYzc