Who does Old Hamlet fight against?Now, there are two answers. One of which is clearly Norway. The other, it would be England. But I do have a point...

Good detective work! And I think you've got something...

Old Hamlet definitely does fight against the Norweigans, the sledded Polacks, and so on - as you know. Though I'm not sure you can assume that a king would always serve in military service. Moreover, I'm not sure you can assume that a king would always fight in a military conflict.

But the real crux is this: the actual line reads like this -

Two months since
Here was a gentleman of Normandy -
I have seen myself and served against the French
And they can well on horseback - but this gallant
Had witchcraft in't...

It was two months ago that Claudius saw the gentlemen from Normandy, but probably many, many years before - as a young man - that he served against the French. So no guarantee that Old Hamlet (not that much Claudius' elder, presumably) would have been king back then. 

But oddly - there is no other reference to this French war in the text. Maybe it was a composite army (an allegiance between France and another country). Maybe they were French mercenaries. Maybe Shakespeare was accidentally or deliberately voicing the anti-French feeling of his time! Who knows?