Multilingual Website without big plugin

Hello,
I want to create a multilingual Website without using a big Plugin like polylang or wpml.

I have my main Domain and Subdomain already and now I am trying to figure out how to Link the single pages to each other in a nice way.
If I add a link in Header or Sidebar it’s always only to the same page but I want the single pages to be connected to each other.
It will be a big Website with far than 1000 pages.

Any ideas?
Thanks