What works surprisingly often is setting language to en-UK instead. Apparently many sites consider en-US to be some default that they can safely ignore, but en-UK to be a deliberate choice. Nowadays I use that not just for the browser, but for my phone and computer too.
I can say that it doesn't work on Google Docs, which uses the IP address's language^, even though my language is set to en-UK. Unless you're logged in, but I often have to use incognito because of another misfeature which is that Google will dox you to other editors if you access a public Google doc while logged in. Googlers reading this comment: please fix this!
^ I wonder how this works in places like Switzerland and Belgium that have more than one language. The notion that IP addresses can be mapped to languages needs to die.
One drawback I remember was that sometimes experimental features in some app are available strictly for the US (i.e. "vanilla") version, presumably simply because lack of localisation blocks it for "other" versions.
(Sadly cannot recall concrete program, but I think it was some web browser.)
I had my language set to en-NZ when I lived in Indonesia; definitely had Google serve pages in Indonesian to me (and double frustratingly, with US-style dates).
