FIRST YOU HAVE TO CHANGE THE LANGUAGE YOU WANT FOR THE AUDIO ON STEAM

For exemple im French, on steam i have set Far Cry 6 language in english because i really like english voice over but i also have the interface in english, with this mod you can separate those 2.

You have to mod the game and it's really easy, like 5 minutes
You have to follow this guide https://downloads.fcmodding.com/faq/install/

Mod Installer link : https://downloads.fcmodding.com/all/mod-installer/ (down the page)
Libertad mod link : https://downloads.fcmodding.com/fc6/libertad-mod/ (down the page)

So follow all the steps of the first link then if you don't want any mod you can uncheck mods in the installer, you just have to check "Language Switcher" on Replacements bar

So you have to select for "Current language" the language you have set on steam for your prefered audio language then you select your main language for the interface on "Desired language", install the mod and then launch the game.

That's all, good job.