My companies website setup similar to how you describe.  Each language (or in our case region) is a seperate child portal to www.xmysite.com.  When a user selects North America they follow the www.xmysite.com path.  When they select Europe then they are directed to www.xmysite.com/europe/.  Each of our portals represent different product which are available end users based on their region.  Portals with different content based on language would also follow the same. 

If your content is the same for each portal just a different locale, why don't you just customize your locale so the end user can switch to what they would like?  If your content isn't the same disregard that statement. 

If you have similar content for both languages, I suggest to use one of the 3rd party ML solutions instead of creating separate portals per language. Have a look at EALO from www.Effority.net, PageLocalization from www.apollo-software.nl and DSLocalizator from dnn.tiendaboliviana.com. You will also need 3rd party ML modules for the content, there are several available (a few for free) from the same developers and e.g. from www.portalfabrik.net