fatal error for ?msls_link_create?

For testing purposes i added the following function as is to my functions.php

/**
 * @param int $display
 * @return MslsLink
 */
function my_msls_link_create( $display ) {
    class MyMslsLink extends MslsLink {
        protected $format_string = '<img src="{src}" alt="{alt}"/> <span>{txt}</span>';
    }
    return new MyMslsLink;
}
add_filter( 'msls_link_create', 'my_msls_link_create' );

I presumed that i could add width and height for the img tag, but this function throws a fatal error:

Fatal error: Uncaught Error: Class 'MslsLink' not found in /Applications/MAMP/testdocs/wp-content/themes/archiv/functions.php:21 Stack trace: #0 /Applications/MAMP/testdocs/wp-includes/class-wp-hook.php(308): my_msls_link_create(2) #1 /Applications/MAMP/testdocs/wp-includes/plugin.php(205): WP_Hook->apply_filters(2, Array) #2 /Applications/MAMP/testdocs/wp-content/plugins/multisite-language-switcher/includes/MslsLink.php(84): apply_filters('msls_link_creat...', 2) #3 /Applications/MAMP/testdocs/wp-content/plugins/multisite-language-switcher/includes/MslsOutput.php(41): lloc\Msls\MslsLink::create(2) #4 /Applications/MAMP/testdocs/wp-content/themes/archiv/functions.php(51): lloc\Msls\MslsOutput->get(2) #5 /Applications/MAMP/testdocs/wp-includes/class-wp-hook.php(308): my_custom_menu_item('<li id="menu-it...', Object(stdClass)) #6 /Applications/MAMP/testdocs/wp-includes/plugin.php(205): WP_Hook->apply_filters('<li id="menu-it...', Array) #7 /Applications/MAMP/testdocs/wp-includes/nav-menu-template.php(273): apply_filters('wp in /Applications/MAMP/testdocs/wp-content/themes/archiv/functions.php on line 21

Is there something i am missing in using ?msls_link_create?? Is my presumption to put the function in my functions.php wrong? By the way, i really appreciate this plugin. It’s a great feature to tie everything and dynamically switch languages. regards theo