The bsquare site was a custom solution, the guy who built it was going to write a tutorial about how he implemented it but he never did write one as far as I know. So I'm not sure what he did but he said he did not modify any mojoportal code, only added custom code in his own files.
Probably he created a UserControl with the dropdown and put the UserControl in the layout.master file of the skin. The dropdown does a postback and sets a cookie and then redirects to a translated page based on the language cookie.
Best,
Joe