If Wiktionary used language subpages, e.g. pizza/en, then these are the exceptions that a hypothetical {{pagename}} or {{basepagename}} would have to recognise as "not subpages". i.e. pages which contain a slash (/).
Only a small number of these "exceptions" would need to be hardcoded in the pagename module, while the rest could be automatically detected because they don't look like language codes (due to capitalization, use of numbers, symbols, etc). The use of an "exceptions" list could have the advantage of turning currently "Unsupported titles" into semi-supported ones, at least by many templates.
There is a possibility of a conflict, e.g. if the word "she" existed in Hebrew or "yes" was used in Norwegian, then she/he and yes/no would be ambiguous. (Or if we had an entry for "g" in Kongo, there would be a conflict with the page g/kg.) But I cannot find any actual existing conflicts.
It would be nice if MediaWiki had a __NOTSUBPAGE__ directive built-in to avoid needing workarounds. E.g. something you could add to 20/20 so that the page would not show breadcrumbs and so 20/20/en would work as expected (with a breadcrumb trail to 20/20).
SQLite database file:
en.wiktionary.org-file-user.xowa (2015-12-01 23:27:02)query:
select page_title from page where page_title like "%/%" and page_namespace = 0;