The Flash plug-in version 11.2 which is the latest version does not support the Arabic language.
It shows the characters separated and oriented from left to right; whereas it has to display them as joined characters and oriented right to left.
Adobe says that they will only provide a security update and nothing else, so that clear bug will be kept for Firefox, Opera and other browsers that depend on the standalone plug-in.
Google Chrome display the language correctly because of it's own built in plug-in.
Best Answer
Since Adobe will not update flash for Linux.
Consider using HTML 5 instead of flash for You Tube. It doesn't have that problem. As you see in the below image, it doesn't have the problem.
1Source:OMG