は usage in this example

Hello!

Could someone please help me understand the difference in usage of は  vs が in this example?

Is there a person who can speak English?
えいご を はなせる ひと は います か?

I would have thought that が should be used:
えいご を はなせる ひと が います か?

It seems to be the only particle now that I'm struggling with.  How does the meaning of the sentence change if I were to use ga?  I understand the topic is a person who speaks English... but isn't and English speaking person the subject, too?  How can I tell which is appropriate?

Thanks so much...

This post might help: http://nihonshock.com/2010/02/particles-the-difference-between-wa-and-ga/