Locate the post slug function

Hello. I have this problem, my posts titles are in Greek language so the post slug is also in greek letters. Every time i change it manually ton english characters but i have this code that it will do it automatically. I do not know where to add it.
Thank you.

function title_slug( $title )
{
$slug = $title;

$bad = array( '?','?','?','?','?','à','á','?','?','?','?','?','è','é','ê','?','ì','í','?','?','?',
'ò','ó','?','?','?','?','ù','ú','?','ü','Y','à','á','a','?','?','?','?','è','é','ê',
'?','ì','í','?','?','?','ò','ó','?','?','?','?','ù','ú','?','ü','y','?',
'T','t','D','e','?','?','?','?','?','μ',
'"',"'",'“','”',"\n","\r",'_');

$good = array( 'S','Z','s','z','Y','A','A','A','A','A','A','C','E','E','E','E','I','I','I','I','N',
'O','O','O','O','O','O','U','U','U','U','Y','a','a','a','a','a','a','c','e','e','e',
'e','i','i','i','i','n','o','o','o','o','o','o','u','u','u','u','y','y',
'TH','th','DH','dh','ss','OE','oe','AE','ae','u',
'','','','','','','-');

// replace strange characters with alphanumeric equivalents
$slug = str_replace( $bad, $good, $slug );

$slug = trim($slug);

// remove any duplicate whitespace, and ensure all characters are alphanumeric
$bad_reg = array('/\s+/','/[^A-Za-z0-9\-]/');
$good_reg = array('-','');
$slug = preg_replace($bad_reg, $good_reg, $slug);

// and lowercase
$slug = strtolower($slug);

return $slug;
}