I think inventions are the way to do it. Have an invention on an early tech that you can only get if you've go the required tags, and tie the unit to that invention.
What you'd do specifically here is add a new invention to whatever allows engineers (muzzle-loaded rifles IIRC) with a base discovery chance of 0% and a modifier of 100% if the country has the tags ENG or FRA, make said invention unlock engineers, and make the tech as a whole not unlock engineers.
I think that's everything, though you may have to fiddle with things at the game's start because I think being given a tech at the game's start might unlock every invention to do with it by default - I'm not sure about that though.
