RTL means right-to-left written languages and when you read this you will read LTR; left-to-right. SharePoint 2010 shipped a couple of Silverlight controls made with the version 3 and that is not aware of RTL so you might need to know where this could be a problem.
One example here with the Create new Site dialog:
If you can read Arabic you will understand that the characters are in the wrong direction. –> noitcerid gnorw…
Why this happens? You created a Site Collection on the Central Admin with a language other than RTL. When you navigate to that Site and choose the New Site Creation dialog Silverlight will be used instead of the HTML user interface. This is a by design behavior.
How to solve that problem? Create a Site Collection with RTL and from that site you will get the HTML user interface for the Create new Site dialog and that is able to show your RTL in the right direction.
Q: I have Silverlight 4 installed and that is able to show RTL but why this does not help?
A: Silverlight 3 controls are compatible with SL4 so the result will be the same.