I updated my EE install from 2.6 to 2.9. I can now see my homepage but when I click on any link I get this:
Unable to load the requested language file: language/english/template_router_lang.php
Solved! Updating instructions say to copy the entire system/language/english folder from the old system folder to the new, updated one. But the old one was missing a file: template_router_lang.php.
I was able to go back to the files I downloaded from EllisLab, find that file, and put it in my server. Voila.