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.

Reference: Android PR, Android task