They have publicly announced that there will be various historic modes or battles where IIRC tanks will be restricted by nationality and in some cases by year of availability. There are also clan wars where clans duke it out over a persistent map.
American tanks (but apparently not TDs) will be available at release. British, French, and Japanese tanks are planned after release, and the German, Russian, and American trees will be extended considerably. They are also talking about adding the Italians, but I think that's more up in the air than the other three.
You can find the
release and full tech trees (i.e., the list of tanks) here.