JMM is getting English down pretty well. It's unfortunate that more great Mexican fighters don't. Little Ricardo Lopez could have made millions more if he had learned English--not that they should have to do so, mind you; it's just that so much of the paying demographic speaks English, and likes to hear them directly without a translator.
If one bases their career on being the voice of a culture, i.e. Duran for Panama or Julio Chavez sr. for Mexico, then this may not apply.
However, on the main, it is a great marketing strategy to be able to talk directly to the largest chunk of ppv buyers without translation