How to add “latin-extended” into a php query for a WordPress theme?

my title doesnt really tell anything. So I’ll try to be clear. I am using a wordpress theme and it has google fonts pre-installed. However the included php for the fonts doesnt contain latin-extended option. How should I add the “&subset=latin,latin-ext” ?

The code follows:

foreach($googlefonts as $googlefont) {

if(!in_array($googlefont, $default)) {
        $themename_customfont = str_replace(' ', '+', $googlefont). ':300,300italic,400,400italic,700,700italic,900,900italic|' . $themename_customfont;
}

}

if ($themename_customfont != "") {  

function google_fonts() {       
    global $themename_customfont;       
    $protocol = is_ssl() ? 'https' : 'http';
    wp_enqueue_style( 'themename-googlefonts', "$protocol://fonts.googleapis.com/css?family=". substr_replace($theretailer_customfont ,"",-1) . "' rel='stylesheet' type='text/css" );
}
add_action( 'wp_enqueue_scripts', 'google_fonts' );