I expect that 20-40 years old, are able to use an English language website. Thus your made up 95% is extremely high.

Yeah, until you hit the parts of the UI that aren't translated from German or have to return stuff and the vendor only communicates in German and the auto-generated emails are in German and the vendor feedback list is in German and th4 reviews are in German...

They're not ordering from amazon.com (US), they're probably ordering from amazon.de with a thin layer of English translations + autotranslations.

If you're setting your preferred language in English on the website, all further on interaction should be in English. (BTW they have multiple language dropdowns that I didn't even highlight because I'm sure old generations do not know how to use those). Lack of English translated terms isn't something I've personally encountered, and for the reviews it's true I scroll a bit further down the page to see the English reviews from .com

I've used .de very often in the past, and when I stopped using Amazon, as said in reply to someone else https://news.ycombinator.com/item?id=31445473 , quality dropped (among other downsides) when they allowed third party sellers and those flood the product listing pages.

With those specific suppliers (and anything that isn't fulfilled by Amazon), I expect to be possible to have the issues you're referring to. But I wouldn't clump that together with Amazon not being available in some EU countries.

When 70-80% of what's sold on amazon.de comes from other vendors and you can't filter those results out, what you're saying doesn't help at all.

Yes, theoretically you're right, but in practice it's useless advice.