If all you want is to "translate" a bullet diameter from one system to the other, there are on-line conversion sites that will do the job for you. But all they will do is tell you that ".38 inch" translates to 9.66 mm. They won't tell you that a .38 Special bullet is really .357" or that .38 Special is not the same as .38 S&W. Or they will tell you that 9mm is .354", but not that 9mm Largo isn't the same as 9mm Steyr, but is the same as 9mm Bergman-Bayard.
If you want to find out what a U.S. cartridge is called in Europe or vice versa (like 7.62x63 is .30-'06) then you need a book like Cartridges of the World.
Jim