I would say the decision is made by Apple depending on the country. For example, in Switzerland, where German, French and Italian are the official languages, you can select among those keyboards. However, interestingly enough, you can also select a US keyboard layout as well.
In Canada, French and English being the official languages, you would expect the two keyboard layouts be available. Then again, for some mysterious reason, Arabic keyboard has been added to the selection. Since when Arabic became so popular in Canada, I have no idea. I would not be surprised to see Chinese given the large Chinese population in Canada but Arabic?
In Turkey, logically speaking, at least the US keyboard layout should be available, similar to Switzerland.
The best thing is do what I do and buy it in the states. The price difference between, say a 12" MB is $1750 in the US (including the NY sales tax) and at todays exchange rate the same Macbook is $2100 in Turkey (all taxes included). Add $250 Apple Care+ on it when you buy it in the states to extend the warranty to 3 years international, now the price is about the same but you have 3 years Apple Care+ warranty instead of 2 years standard warranty if you buy it in Turkey.
The only problem is, either you have to be traveling to the US or have someone bring it for you.
The Turkish QWERTY keyboard layout is not entirely unliveable. I mainly object to the small Return key and the special character layout being completely different on top of the numeric keys and the @ sign being hidden (alt-Q and there is no way to guess if you don't know where it is).
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Sounds like a good reason to have a trip to Vegas! Buy the laptop while you are out there...
NY is closer but not as much fun