How do I add a new language?
Adding full language support for a device is done at rom level, the languages that the default keyboard will support are added before compilation and the android keyboard will only give you the option to download dictionaries for the languages it supports.
A good workaround is to download a keyboard app (such as GOKeyboard) that supports adding languages unto it.

Locale changing apps are nice but they may not be as thorough in their translation as a rom that was made or specially translated to a specific/your desired language. If your language is Bi-Di (Bidirectional, such as hebrew or arab) and the rom doesn't support it, it may introduce bugs as well!

The best solution will be to flash a new rom that holds your desired language, either a stock rom made for your area by your device manufacturer or a custom rom, Cyanogen is good because it probably supports every language that complies to the unicode standard (including turkish).