not sure why you can’t do both to be honest. Make your website templates RWD and it doesn’t matter what language you put it in.
But if you HAVE to choose then it depends on what you are selling and who you want to read it. If your user base is entirely English speaking mobile users then having a desktop website in Cantonese is not going to be very useful.
From an SEO point of view google does rank mobile friendly websites higher on mobile searches and the majority of the Western world speaks english so if that was where i was aiming at i’d make a good RWD english site first.