how can I view support sites in English instead of being forced into local language?

Not a solution, but a potential workaround: Use Safari private browsing.

Private browsing keeps all the cookies isolated, and makes it much easier to hack the Apple URL from some localized version to en-us or en-uk or whatever other preferred language.

With the cookies captured within the context of private browsing, the too-smart-for-its-own-good design can be more amendable to user-applied URL changes.

I use this method routinely when editing Apple Support URLs, as the Apple-implemented language-selection mechanism is incredibly sticky otherwise.

Web sites are built by programmers, and programmers sometimes don't understand how normal people think.

That "locale selector" at the bottom of the page isn't really for selecting a language. That control is for selecting a "locale" in the legal, business sense of the word. (There is another, technical computer definition of "locale" too, but that doesn't apply here.) The idea is to get you to an Apple web site that is appropriate for your country, tax collection, and legal issues. In some cases, you might get a change of language too. But that's strictly a side effect. That control simply isn't for changing the language.

Apple's web pages look at a low-level HTTP header field to determine what language to show you. You can override this by hacking the URL as you have already done. You can also sometimes override this by using that "locale selector". But that won't stick because Apple doesn't always include the same language specifiers in the URL. Sometimes Apple omits those knowing that the automatic language selection will properly handle the language. But if you've already hacked that, one way or another, then you'll lose your hacked language eventually and get frustrated with that in a hurry.

What you have to do is set the language that Safari uses. By default, Safari uses your system language. You can override this in System Settings > General > Language & Region and scroll down to Applications.

Unfortunately, if you only want one specific web site in a different language, then you have a problem. It's all or nothing. One possible solution is to download Apple's Safari Technology Preview and use that instead. It's like having another version of Safari. For complicated reasons, I'm not certain this will work. If it doesn't work, a guaranteed solution would be to use Google Chrome. Normally I don't recommend Google Chrome. This is literally the only thing I ever use Google Chrome for. In Chrome, you can go into the settings and specify which languages you want to support. In my case, I have Chrome setup so I can easily switch to French and test my websites in French.

Try switching to en-uk instead of en-us. This worked for me once, no idea why.

Consider getting this before an Apple employee using this link:

Feedback - Apple.com Website - Apple

Be sure to use the first option the the feedback type pulldown as the others do not work correctly:

We are your fellow end users and can only offer suggestions; we cannot fix anything.

Unfortunately sometimes the Apple site will just not be able to display certain language versions of Apple's articles on a particular day. There have been times when I am unable to access the US versions of an Apple article and will be redirected to some other region (the article is in English though) even when I try to force it with "en-us" or just removing that section of the URL to attempt to automatically acquire the local version.

I agree some apps & sites are just too "smart" for their own good.