Do you have any good resources for demographics of the various states of the time?
You can speak polish right? Mind if i recommend or even send over some books by Chomik or by mail?
Poland at the time of Grunwald battle had approximately 2.7 million people, Lithuania had around 350.000, Teutonic Order had approximately 650.0000.
While we dont know the exact numbers for both sides we do know the composition of the guest/mercenary forces for the Teutonic Order which constituted of 3000 French/Italian knights, 600 Czech infantry and 200 Welsh longbowmen. thats at most 4000 soldiers in an army at least 25.000 large (probably closer or above 30.000) the remainder were Germans.
There was also an indeterminate number of german guests but given that Malborg ran out of rooms a week into two month assembly and it had prepared dormitories for 3000 knights we can assume that they formed the vast bulk of guest cavalry.
Practically 90-95% of all knights and infantry who fought at Grunwald for TO were Germans since we know French fought Poles at the battle of Koronowo and no heavy cavalry survivded Grunwald on the German side so French couldnt have taken part since they obviously werent dead or captured.
As for Polish side, Poles constituted 100% of the heavy cavalry, 100% of the heavy infantry and more then half of light cavalry, Lithuanians having a population of on the polish voivodships wouldnt be able to field more than 6000-7000 men especially since they were in conflict with Novgorod at the time.
So this was most definitely a Polish-German battle, not only did Poles and Germans constitute the crushing majority of forces, commanders on both sides were polish and german (with the exclusion of Jagiełło) and all the fighting was done by ethnic polish and german units.