tml_ms_signup_get_available_languages()

function tml_ms_signup_get_available_languages() {
    /** This filter is documented in wp-signup.php */
    $languages = (array) apply_filters( 'signup_get_available_languages', get_available_languages() );

    /*
     * Strip any non-installed languages and return.
     *
     * Re-call get_available_languages() here in case a language pack was installed
     * in a callback hooked to the 'signup_get_available_languages' filter before this point.
     */
    return array_intersect_assoc( $languages, get_available_languages() );
}

View src/includes/ms-functions.php, line 1165 on GitHub