On Special:TranslatorSignup, users can sign up for only English, too. That doesn't really make sense, but it's a valid use case. At least then we know they cannot help in making translations :).
However, it doesn't make sense to ask them to visit something like , as they'll get the message "English is the source language.".
(1) Translators signed up for a site's content language (or later possibly the notified page's language) should not be notified. They can of course get a notification in English for another language they signed up for, because that's their primary language.