Stuff like .../?hl=en may help indeed (but it has to be put manually.
(I assume it stands for "No Country Recognition" but I'm unsure of that). Visit it once and I believe it stores a permanent cookie on your computer telling google not to re-direct you to a country specific site.
Been using it for about 10yrs now as it annoys me when I get re-directed when accessing sites over a vpn.
My Firefox sends the following Accept-Language: "en-us,en;q=0.8,sv-se;q=0.5,sv;q=0.3".
When I go to Google.com in a new private browsing context, I am redirected to Google.se with English language. When I go straight to Google.se in a new private browsing context, the page is in English.
Ads though. Still in German. Not sure if that is a good thing or a bad thing. I'm even more blind to them in the wrong language. I guess that is a good thing!
As someone living in the US who speaks English and Japanese, I often get annoyed by the fact changing the language would affect feature set to match the language and vice-versa. I almost want something like ja-US and en-JP!