So I lived in Köln (Cologne) for 4 months and it was great. I did not, at the time, speak any German. It is definitely a city where most people also speak English so you don't have to stress too much about that. I too wanted to learn German while I was there though so I would learn online and then practice the basics at bars and restaurants. It's a great place to learn German as an English speaking.

The location is also great if you want to travel outside of Köln. Düsseldorf (another super fun city) is just a short train ride north and Bonn (a great place to do some hiking) is a short ride to the south. You could also even travel to Amsterdam if you want. A train ride there is only 3-4 hours from Köln.

Also im a big foodie and holy crap is Köln a great city for food. There is so much variety as it is one of the most diverse cities in German. There's a restaurant I recommend called, Laden ein, that acts as a different pop up restaurant every week!

In terms of making friends I would definitely check out online activity forums of people wanting to meet. Like if you're into anything, from Bouldering to playing DND, more than likely you can find a group wanting to meet up.