I've been reporting this issue for several years under radar 45431755 and now FB FB5708192.
Ever since the launch of tvOS Siri has been disabled if you are using an iTunes store in a different language than the Apple TV interface.
Yet when the HomePod launched, which now also runs tvOS, Siri was allowed with a Dutch iTunes Store account. So why isn't this allowed on an Apple TV?
In my case, the Apple TV is set to English (United States) and I'm using a Dutch iTunes Store account since you have to use the store in your native currency.
All our devices here at home are in English (United States) and even the home accessories names are in English in order to use them with Siri on iPhone, Apple Watch, Mac, and HomePod... But not on Apple TV because it is not possible.