Just curious, why would you be concerned with the US NEC on a project overseas? That's why the NEC doesn't address voltages found in other countries by the way. I would think it a better idea to find out what standards are used in the destination country and go by that. For example a lot of counties use British Standards for things like this, especially former British colonies like India, Pakistan and several mid-east countries.
You might see if your library has a copy of this book, "
World Industrial Standards Speedy Finder". I've found it at a local University library, pretty useful for things like this. It cross references US standards to applicable standards in other countries.
Barring all that, if you are supplying from here based on a 460V 60Hz motor, when using it at 400V 50Hz, the current is not going to be much different because the motor is going to spin 20% slower at 50Hz. So the pump will be doing at least 20% less work and if it's a centrifugal, it will be doing only 64% of the work it would do at 60Hz. I would thing if you sized the conductors based on 460/60, they will be over sized for the new application. So the only other concern is the SCPD for the motor windings, and being that Fire Pumps are "run to destruction" anyway, I don't see a concern if based on the 460/60 values.