Similar to T117763 there could be a super trivial guessing game going on in the siteselector (client-side JavaScript). When you paste a full URL (for example https://de.wikipedia.org/wiki/Heinrich_Kaminski) and this gets you an empty result set, a super-trivial regular expression like /\W(\w+)/ could extract the subdomain and make a second guess based on that. This regex searches for the first word that is preceded by a non-word character and would find the word "en" in (en), //en.wikipedia.org and other strings like foo en bar.
This could be a first step towards T49983: sitelinks ui: accept a link instead of a value from the dropdown box (also see the parent task T112241).