How can I change spell check to Turkish in Pages?
Normally, for a language not included in OS X, you can try adding a dictionary from the OpenOffice app, as explained at

http://m10lmac.blogspot.com/2011/06/extra-spell-checking-dictionaries-for.html

Unfortunately, Turkish is not there.  Here is a a site with some .dic and .aff files which I have not tested myself.

http://code.google.com/p/tr-spell/downloads/list