I’d always suggest English as the first choice 
Seriously though, alot depends not only on your traffic, but what content you have and how it is best aimed. If its a casino or poker reviews site and you have rooms/casinos with French translations, then that makes some sense…etc etc.
Outside of English and Chinese, Spanish and French are widely spoken outside their native lands (South America, Africa, Canada etc). That’s where I started. Or you could look a it from a cultural angle: which nationailties have a gambling culture. Turkey? Greece? There are several considerations.