Agreed with Inzil. Two explanations: Tolkien had the backstory in mind when picking out names (e.g. the story of Helm Hammerhand who wintered in this keep) and used fitting names, or he reeeeally liked those two places on the Jutland map and made the backstory match the names. I think the former is the simpler explanation, without being a Tolkien expert of any kind.
A google search tells me that there actually is a River Isen in Bavaria, so if there really is a specific geographical inspiration for Saruman's abode, Germany might win this round.
Personally, I tend to see coincidence in the opposite direction. Like why is there a town in Italy named after Hurin's eldest. And the poor Dunedin, which I always feel the need to spell correctly, with an "a" after the "d".
Not every sound-alike or look-alike is a real reference. So unless someone finds evidence that Tolkien actually had more affinity for eastern Jutland's geography than the average map, I will remain dubious about the influence here.