The devs will tell you more about this, of course, but as far as I know, the foreign troops are intended to be a new reward doctrine (a doctrine you can activate in the Rewards menu and that will replace one of the 3 basic doctrine trees). Not much is known about what's going to be in this doctrine as it's still in the works (and most probably not going to make it into the upcoming patch), but we already know that Pz 38(t) (that is to be removed from Support doctrine) will be one of the unlocks for the Foreign Troops doctrine.