I am working on creating 2 specific site for English speaking and Spanish speaking audiences.  www.mydomain.com/English and www.mydomain.com/spanish.  How can I set the Spanish portal to be fully in Spanish language?  The menu bar, the registrations should all be in Spanish (obviously without affecting the English portal).  Can someone help me please? 

You will want to "localize" the Spanish site. Start with the Spanish language pack. Then, you'll want to look at the various modules that you are using and localize those.

I believe that there are some third-party tools for localization, but I can't speak with any authority about them.