My internet has been super slow lately and some websites are loading in German for some reason, even though I am in the UK. I suspect my ISP has messed up the routing. How can I check where my connection is originating from to prove it to tech support?

Bad routing causes weird language issues and lag. You need to check the geo-data of your connection. I use the tools on mylocation.is to check my ip location and ISP info. It shows a map of where your IP is registered. If it places you in a different country, you have solid proof to show your provider that their routing tables are broken.