SecurePoll has its own auth and i18n system which uses the language the user specified in their authentication source domain. It doesn't override $wgLang so typically the sidebar will be in English whereas the content area will be in the user's language.
Users voting with an RTL language will see a broken and unreadable intro and voting form due to the incorrect embedding direction.
MediaWiki has a concept of page content language which basically does what we want, except that there is some associated tech debt which makes it difficult to use from a special page.