To match Android, we should send in the legacy language variant code (like zh-tw) to the read more langCode parameter in the footer setup Javascript. This logic can be added here. The legacy language variant can be found by referencing articleURL.wmf_languageVariantCode to get the modern language variant code, then checking it against WikipediaLookup.allLanguageVariants to grab the equivalent legacy language variant code.
While we're here, we should look into why "About this article" and "Read more" display as English as see if this change fixes it:
Reference: Android PR, Android task