Yes, you can omit "who is".
However, it doesn't make sense. Look at "frequently". Is the person looking for "people who can speak English frequently"? If so, that's a strange request. People don't have the ability to do something frequently. They have the ability to do something and then they either do it frequently or they don't. Or does "frequently" refer back to "need"? Frequently, I need anyone able to speak English.
Student or Learner