Hi Davico,
I recommend using Visual Studio's object browser to get a grasp on an unfamiliar class and learn about its functionality. I find that using it is a lot more intuitive and straightforward than browsing source code.
To modify the language on a per-request basis, you're likely going to modify either System.Threading.Thread.CurrentThread.CurrentCulture or System.Threading.Thread.CurrentThread.CurrentUICulture to accomplish your task. Although DNN implements its own custom localization, I'm virtually certain that it still uses these values for its internal functionality.
To modify the default portal language, look to the PortalSettings.DefaultLanguage property.
If you're looking to do something different than either of the routes I outlined above, you'll probably need to elaborate on your goals a little further :)
Hope this points you in the right direction!
Brandon