I've seen this question a few times, so I figured I'd post a sticky thread. None of this is saber specific, but there are some general methods to cleaning nickel plating without damaging the finish.
https://m.wikihow.com/Clean-Nickel-PlatingI would suggest buffing it down with soap and water(method one above) before trying any chemicals. Just be sure to use WARM(not hot) water, gentle dish soap(I'd recommend Dawn), a non abrasive cloth, and gentle circular motions. This method should be enough to remove the fingerprints often associated with plated hilts, I wouldn't imagine them getting soiled enough to where chemicals are needed.