On an allied boat : if you use allied sailors (from another country) they will work as they should except that only rookies and experts will be counted (ie : about 60 - 70% effective form a normal VE sailor). Axis salors on allied boats won't work effectively.
On an axis boat : same logic than previously, just use the opposite way.
Exception with MN sailors who can work on both Allied and Axis ships. I don't know if it's the same case with RN and SN sailors.
For example, With a typical 120MN crew, I can reach the OH speed cap of any BB4 (but doesn't last very long).
Exp is not affected by the nation but by the boat as said in others posts ;)