Help with changing template language depending on post language


I want to have a blog with posts in English and German. I read and followed this guide and have a /de/ collection page for German posts now. It would be nice if when I open a German post, the whole interface could be in German as well. I tried what’s described in step 2 of the guide above but that didn’t have any effect.

I tried modifying the Attila theme, which has German translation support, but the interface still appears in English. I made a copy of default.hbs called default-de.hbs and changed the <html lang="{{@site.lang}}"> to en and de respectively as suggested in this post but I’m not sure how to proceed from there.

I tried changing my post.hbs from version A (which didn’t work) to version B:
Screenshot 2020-04-07 at 00.15.01
(had to combine because I can only put 1 image in the post)

Needless to say this also didn’t have any apparent effect on the post language of my example post tagged with #de.

I have some basic knowledge of programming, html and css but none with JS and Handlebars (I found an easy way to install a ghost server on Heroku) so I’m super lost. I know that the {{!< default}} part fills the whole code into the body of default.hbs but I don’t know if this syntax above makes any sense or how I could make it make sense. Grateful for any help, I think a hair or two of mine might have turned grey trying to set up a nice multilingual blog :disappointed_relieved: