Adding a menu item with a redirect to another site

Hi,

I’m currently building a site which will be in english and in french.
I’m using a plug in which allows the user to select their language by clicking on the corresponding flag.
This however isn’t showing using androids and iphones.

I would need to add a link into the menu bar with the words “Francais” in one and “English” in the other so that people can swap between the 2.

Sites are: https://www.social-be.be (english) and https://www.social-be.be/fr (french).

Anyone knows how to do this?

Many thanks!