Hi guys,
first of all, sorry for my english, I'm not native.

After Joomla upgrade doesn't work language filter properly, specifically Home page. If you switch to another language, lets say from english to arabic (www.yourdomain.com/ar) and then you press home button, you get back english version (www.yourdomain.com) without "ar" on the end.
I guess It's issue somewhere in the template, if you switch to the origin template PROTOSTAR, the issue simply disappear. I tried overwrite language filter files from version 3.3.6 and after that it's working. So, I thought you should know. It could be incompatibility problem of many of your templates.
Tested on your Kay Quickstart, so you can try on your own.

Sincerely. Richard Vošický.