lang should be an ISO 2 letter country code (e.g. en, de, fr). This is used on the web to set the language of the entire post.

rtl should be the string "true" if phone settings are on an RTL language, otherwise "false". This is used on the web to determine if the entire post should be displayed LTR or RTL. It might get removed from the API in the future, but was added mostly so the user could potentially override RTL display with an app setting. Open to discussing.