From you code example it is not clear what you are trying to do. Of course Mathematica is not Python, so writing (what I suspect is) Python code in Mathematica will just lead to tears. What is dictionary? What is ArabicLetters? Have you defined these somewhere else? What is x? Of course the syntax in your expression (with the colons) is completely non-Mathematica. So, first things first, take a moment out to learn the syntax of Mathematica. Here is a start:

http://www.wolfram.com/language/fast-introduction-for-programmers/interactive-usage/

If you are using Mathematica version 10, you might want to look up the documentation for Association as that might get you started on the road to what I think you are wanting:

http://reference.wolfram.com/language/ref/Association.html