function replace_howdy($wp_admin_bar)
{
    $my_account = $wp_admin_bar->get_node("my-account");
    $greeting = str_replace("Willkommen,", "Hallo", $my_account->title);
    $wp_admin_bar->add_node([
        "id" => "my-account",
        "title" => $greeting,
    ]);
}
add_filter("admin_bar_menu", "replace_howdy", 25);