This issue applies to pages in the situation that results from bug
XWIKI-15127, namely pages that have the default language empty and a translation in french , in a wiki which is set with supported languages en and fr and default language en.
Set the wiki to french and navigate to such a page, you will see its french version.
its language switcher in the drawer shows french language, and only that.
its language switcher in the drawer should show the french language (currently viewed) but also the option to see the default version of this document.
If the language of the wiki is set to English, both default language and french language are displayed in the switcher - normal because the displayed version of the document is the empty language version - and current language is always displayed in the switcher - and french is an available translation.
A priori, the reason for this is that the empty language, which is an "available" language for the document is not a language that is available on the wiki (only en and fr are configured on the wiki). However, since empty language cannot be configured as an available language on the wiki, there is no way to make that version of the document show. In addition, I think the original version of the document should always be available in the language switcher, regardless of whether it is or not configured in the available languages of the wiki.