@TerminalAutism It's not only used by Japanese people (the only reason I know is because of people linking to it on forums, soycial media, blogs, and so on).
Invidious has user accounts, and the "popular" tab is basically everyone the users are subscribed to.
Since that one has only 1 account, and that person is a monolingual Japanese frog, the "popular" tab is going to show only Japanese content.