Change language on current page

Hey there, I've build a small box with a german and great britain flag in a Twig Template which I build in on every page.

Now I would like to change the language of the current page if I click on one of the flags.

So I thought href="en" would be the solution. It works fine on the main site, but when I'm on a subpage I get a 404 because of a double language ending in the URL like "www.domain.com/en/en". Is there any possibility that I can set a link with a variable for like "page I'm currently at" and just change the language ending?