Tajik is a variety of Farsi/Persian spoken primarily in Tajikistan and Uzbekistan, but also Iran and Afghanistan. In the former two countries, it uses the Cyrillic alphabet with 6 characters beyond those found in Russian. OS X includes the fonts necessary for this language.
You can get experimental Cyrillic Tajik keyboard layouts here. The QWERTY version has a graphic to show the character location. The PC version has the special Tajik characters on the Alt/Option layer of the normal Russian keys. Leave a comment if you find errors or think improvements are desirable.