Replace form field error text?

I actually managed to chage it with this:

function replace_registration_site_name_error( $translated_text, $text, $domain ) {
$default="Site names can only contain lowercase letters (a-z) and numbers.";
if ( $translated_text == $default ) {
    $translated_text = __( 'Test', 'my_text_domain' );
}
return $translated_text;
}

This works great.
However I am not able to change these two strings with this method?

Site name must be at least %s character.
Site name must be at least %s characters.

Anyone know why?