Download the Turkish language pack for K2 here:
getk2.org/assets/translations/download.php?language=tr
Make sure the Turkish language pack for Joomla is also installed.
Switch your site's primary language to Turkish.
If the above don't work, perhaps portions of your site (e.g. K2 template overrides) don't use language strings at all and thus you only see English as the primary language displayed.