Not all extensions support other languages like Turkish.
If that isn't the case then they usually revert to the English locale.

Thank you for your support. But Firefox's old versions all plugins was in Turkish. I am using Firefox 4 now.

What is the value of the pref general.useragent.locale on the about:config ?

Make sure that it is set the the language code of your locale (tr).

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.
You can use the Filter bar at to top of the about:config page to locate a pref more easily.