Put in functions.php

https://wordpress.org/support/topic/adding-tags-with-commas-in-them

// filter for tags with comma
//  replace '--' with ', ' in the output - allow tags with comma this way
//  e.g. save tag as "Fox--Peter" but display thx 2 filters like "Fox, Peter"
if(!is_admin()){ // make sure the filters are only called in the frontend
    function comma_tag_filter($tag_arr){
        $tag_arr_new = $tag_arr;
        if($tag_arr->taxonomy == 'post_tag' && strpos($tag_arr->name, '--')){
            $tag_arr_new->name = str_replace('--',', ',$tag_arr->name);
        }
        return $tag_arr_new;    
    }
    add_filter('get_post_tag', 'comma_tag_filter');
    function comma_tags_filter($tags_arr){
        $tags_arr_new = array();
        foreach($tags_arr as $tag_arr){
            $tags_arr_new[] = comma_tag_filter($tag_arr);
        }
        return $tags_arr_new;
    }
    add_filter('get_terms', 'comma_tags_filter');
    add_filter('get_the_terms', 'comma_tags_filter');
}