We'll we had a French Captain (albeit with a British accent) command the Enterprise in TNG. Riker was about the only American Character in the main cast. And Sisko was black and a Starbase/starship Commander. If you go further into fan creations such as a PBEM's you see a wide diversity of Characters.
ship captain may not be too far out of the question. A Russian
ship captain would probably be pushing things a bit far.