What is the best way to set up links between pages in a 2-site multisite install?

I need to mirror the site in both English and French.

I have set up a two-site multisite install and need some advice on how to go about adding links to the header of each page to switch to the other language.

I cannot hard-code it into the theme, as there wil be updates which will likely break it.

Have you got a plugin or even any advice for me?

Thanks.