TL;DR I exclusively use English for everything around Discourse.
Everything in my browser is set to English (US).
My user profile language on Meta is also English.
My browser sends Accept-Language: en-Us,en;q=0.5 and nothing else.
My search term for the Google screenshot I provided in this topic were “How to have multiple copies of default theme”, because I was explicitly targeting that topic in order to make the screenshot. My original search term was “discourse duplicate theme”, and that was the search result that led me to the first screenshot (you can still see the search term in the top right there).
As you can see in the screenshot of the Google search results page, Google also offers me a mixture of English and Dutch. It’s probably doing that because I am physically located in the Netherlands. When I travel to Germany it will give me a mixture of English and German. My theory is that the ?tl=nl search result might have been slipped in by Google because of my location (which is stupid but yeah). Still, I don’t expect any Dutch language on Meta when I am logged in.