for example, when players unlock tanks in the full game, will we see a variety of tank designs to fit their culture, like for Germany we get the leopard, America, we get the abrams, England, the cheftain, and French the amx series?

Generic units have unique aesthetic depending the culture in humankind : when it's the generic units of the era of the related cultures.

So, generic Japanese Arquebusiers will look different from Korean Arquebusiers.

But if you trenscend Japanese or Korean, their industrial line infantry will probably have the same east asian aesthetic.

still, their Early Modern naval ships aesthetic are shared. A caravel stay a caravel.

I can't speak for contemporary era.

Yes, just like @Narcisse explained. I was really amazed when I first notoiced this in the early OpenDev that Norsemen swordsmen looked different from the French etc. Just like Narcisse also said Japanese Arqebusiers looked like Ashigaru, while Korean looked different. Very cool indeed. So yes, not only emeblmatic units will look different, but also alot of the regular units. However if You have for example Norse sworsdmen and then change to another culture in next era, Your units will adopt the new cultures look. Only embelamtic units will look the same until You upgrade them and then they too will change to the new cultures look. 

This pretty much makes mods like Civ 5 have called R.E.D. (short for regiment ethnic diversity) or any Ethnic diversity mods are more or less reduntant for Humankind. I always used to play Civ 5 with either one of such mods to make each Civilization look different, since it added to immerson and looked nice. With Humankind I more or less won't need such a mod since it's already built into the game. Unless some mod want to add even more units of course. No idea what type of mods or what functions in mods Humankind will support yet. All I think that has been said is that it will most likely support modding, but maybe not at launch, or maybe not full support at launch, just limited support with more to be added later. I don't think alot of info on that part has been announced yet. I've been off-the-grid for a while, so I am trying to catch up on all info. If someone has more info on that please add it, or correct me if I'm wrong.