Help to change the text for new website notification (wpmu_welcome_notification)

Here’s an untested example, how you could modify the email body to your needs:

add_filter( 'update_welcome_email', 
    function( $welcome_email, $blog_id, $user_id, $password, $title, $meta )
    {
        // Override the email body:
        $welcome_email = __( 'Dear User,

Your new SITE_NAME site has been successfully set up at:
BLOG_URL

You can log in to the administrator account with the following information:

Username: USERNAME
Password: PASSWORD
Log in here: BLOG_URLwp-login.php

We hope you enjoy your new site. Thanks!

--The Team @ SITE_NAME' );

        $current_site = get_current_site();
        $url          = get_blogaddress_by_id($blog_id);
        $user         = get_userdata( $user_id );

        return str_replace( 
            array( 'SITE_NAME', 'BLOG_TITLE', 'BLOG_URL', 'USERNAME', 'PASSWORD' ),
            array( $current_site->site_name, $title, $url, $user->user_login, $password ),
            $welcome_email 
        );
    }
, 10, 6 );