How can I have a URL like www.example.com/about?language=ja to set Japanese for the page?
just to be sure:
at admin/config/regional/language/admin_language, enable "Session" Detection method
make sure request/get parameter set to language
make sure you have about page in 'ja' language

try to visit yoursite.com/?language=japanese, it should be translating interface/etc - this way you can check translation is working.