I found my self
//write the link tag with the $this->google_fontlist
function link_google_font()
{
if(empty($this->google_fontlist)) return;

$this->extra_output .= “\n<!– google webfont font replacement –>\n”;
$this->extra_output .= “<link rel=’stylesheet’ id=’avia-google-webfont’ href=’//fonts.googleapis.com/css?family=”.apply_filters(‘avf_google_fontlist’, $this->google_fontlist).”&subset=latin,vietnamese”.”‘ type=’text/css’ media=’all’/> \n”;
}
In framework/php/class-style-generator.php
Thanks!