Depends on what you want. If you want a layout for people who type both English and Arabic, then maybe a phonetically translated Colemak will work well enough (see my sig topics for examples using Greek/Hebrew/Kyrillic scripts).

If you want a properly optimized layout for Arabic, then you don't want Colemak at all and substituting by frequency certainly won't help you as it's very much about letter combinations (bigrams, trigrams). In this case, you should get a layout generator to work with arabic letters and run a corpus of what you want to type through it.

Which letters are typed in sequence is just as important as how frequent each letter is individually. When I type, I really type letter combinations rather than individual key presses for the most part. That's why the n-grams are important.