Adding on to answer your second question. If you look at a plugin like WPML or something similar, they are actually creating duplicate content in many cases. So a site might be
mydomain.com/en for English and
mydomain.com/es for Spanish. Each endpoint is cached separately. Some cookies are probably set as well, but when you switch languages with WMPL, you are actually redirected to a different page, not the same page with different language content... if that makes sense?