The iOS app currently maps BCP 47 language codes to language codes that MediaWiki understood. Now, MediaWiki supports BCP 47 codes due to https://gerrit.wikimedia.org/r/c/mediawiki/core/+/460038
The iOS app should be able to remove the mapping defined by MediaWikiAcceptLanguageMapping.json and send the list defined by [NSLocale preferredLanguages]. There would potentially need to be updates to wmf_languageBundleNameForWikipediaLanguage as well.