What do you mean by "Amazon is not available in all EU countries". Do you mean like a country specific TLD? Because that is true, but order and delivery is not a problem from any EU country as far as I know.
In my experience a significant part of Amazon's inventory isn't something they'll send outside of the "domain country", e.g. trying to send from .de or .uk (this was before Brexit) to .nl.
It just comes down to suppliers, who aren't serving customers outside of select markets for whatever reason.
I see that as a "market platform" problem. I used extensively amazon.de/co.uk with deliveries in Romania, in early 2010's for a bunch of things. But since then they also opened up their market to any seller, quality dropped, shipment became preferential, or 1 cart could result in 2 separate shipments.
Aside. I've seen the exact same problems with our local Amazon "competitor". As soon as they became a platform marketplace I've started using them less and less because of the same quality/delivery issues.
It's not really a problem of the platform, except insofar as you'd like Amazon to only ship from their own warehouses. It's just Amazon reflecting reality on the ground in the EU.
Which is that even if it's legally a single market it's common that stores that deliver something to your door only sell their products in their own native country, or only within their local region.
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.