Change language of wordpress

Hi!

Can anyone help me, i’m changing the wordpress language using this code: setlocale(LC_ALL, “ko_KR”); but its not working. Can someone teach what is the right code for changing wordpress language in my custom theme. i dont want to change the wordpress language in the admin panel, or in the wp-config.php file. I want in to change on my theme functions.php please help me.

thanks!