For questions, (like this one), where multiple posts use tags like #English or #日本語 to provide multiple language versions, if I click on the language toggle (eg "English" to switch to the English version of a post), the actual contents of other multi-language posts disappear entirely. However, the meta-information and language toggle buttons remain on these other posts.
Work-around: clicking one of the language buttons of a "disappeared" post will restore its contents.
More details: For "disappeared" posts, none of the language buttons is underlined, which suggests maybe the problem is that post is being rendered in a "null" language for which there is no data.