I know that this is an old topic, but because it seems that there is no solutions (Or I didn’t used the right keywords) for this problem, I thought that I could write one. Mostly because I just spent couple of hours trying to fix this same problem from my customers site.
First – remove all pll_ stuff from your functions.php. If you have lots of stuff there, copy paste those to notepad, because you can put those back soon.
Now you should be able to use wp_admin site again. Login and uninstall Polylang.
Install Polylang from plugin repository again.
Restore pll_ stuff to functions.php
Your site should work again.
