Hi,
This text string should already be translated, but you can add the following string to the overrride part of the language file via the menu Configuration > Languages.
LOGIN_REQUIRED_FOR_WISHLISTS="You must be logged in to use the wishlists."
It depend on your configuration, how did you set the parameter when a product is added to the cart in Configuration > Main > Cart ? If it's popup mode, the message should be displayed, if it's redirect to checkout no messages are displayed but it show the wishlists, and if it's stay on the current page, the wishlist module is refreshed.
PS: many fixes have been made recently about the wishlists and cart, so the changes will be in the next release.