Multilinugual, also in WordPress menu

Hi all

I want to create a website that is bilingual with a German and an English flag on every page to switch to the other language version of the site. I know there are lots of plugins, but I want everything to by bilingual, even the WordPress menu.

The only solution I could think of would be to set up two WordPress installations. That would be fine, I would only need to solve one problem: Is there a way to add a link to the theme to the same page in the German version?

Example:
On https://www.domain.com/en/contact I want to set a link to https://www.domain.com/de/contact. This should be automated, e.g. with the page ID or with the permalink.

Is there a way to add a link like this:
https://www.domain.com/en/(same page slug as current page)

I hope you understand the problem and thanks for your help!