Wealh is also the origin of the name "Wales," which is what the English call the country to the west of England; to the Welsh -- who bitterly resented being called foreigners in their own homeland -- it would be Cymraeg, of course, and themselves the Cymry.
Anyway, the Anglo-Saxons had never seen a walnut before, so they called it what they called it. When the things got exported back to the Continent, the Germans had never seen them, either, so they just made the English word into a native German one. Just to check on this new insight, I looked up Walnuß in my pictorial German-English dictionary, and there, behold, Walnuß was glossed as *drum roll* eine welsche Nuß.
So apparently, lots of people knew that walnuts were considered Welsh in origin -- except I didn't. But I do, now.