You should really take a look at Justif-Josef Angel's blog, where he presents some brilliant work that he has done with Silverlight, in particular his work on supporting Right-To-Left languages such as Arabic and Hebrew.
These are more of a challenge than just going Right-To-Left, as numbers will go Left-To-Right, leading to the term 'bidirectional' or 'bidi' being used for rendering text. Take a look here for more details.
You can see his Justin's full Blog Posting here where he outlines his bidi solution for Silverlight 1.0, including samples, webcasts and links to the downloads from codeplex.
More on Bidi here 🙂 [Download the Wav linked on that page to see what I mean...]