Navigation Menu Internationalisation


I am currently trying to add more languages to our blog (it’s a German blog and I want to add English as an alternative). I worked with the instructions stated here: Tutorial: A complete guide to translation & multi-language content in Ghost. The blog is based on the default casper-theme.
Now I do have a two base routes (/{slug} and /en/{slug}). That seems to work.
The problem has to do with the navigation links. Those are absolute paths to some tags (/tag/{tag}) and therefore are always referring to the German routes. My goal is to change those links when using the English version (/en/{slug}), so that you keep getting English posts and are not redirected to German posts (by omitting the /en/ in the path). I have absolutely no idea how to achieve that.
Do you have any ideas how to refer to the right language paths in the navigation menu?

Thanks in advance!