Once you have language support open, you need to drag the language you want up to the top of the list. I found this confusing at first since the other languages are gray. But they are not "grayed out". You can still drag them up. Also the languages you installed will be at the bottom of the list and scrolled of the visible portion of the list box. I had also thought they were not installed.
Drag the one you want to the top of the list and click "Apply system wide". On the Regional Formats tab, choose the language you want (here it's a pop-up list and easier to use) and again click Apply system wide.
If you plan to use switch between all three languages frequently, you might want to take a different approach. Changing from, say, Turkish to French will cause the system to want to change the names of your home folders. A slow-down at best but a possible a source of problems later.
My recommendation is to make 3 new users, one for each language. user-turkish, user-french, etc. When you create the user, set the language to the one you want. This will have the same effect of changing the language/regional settings but for that user only. After that you can edit permissions on the home folders to make sure you can use all of your files.