The rise and fall of great and less great nations.

[Bubblegum Knight]

Hi. Me again. Why and how are empires formed? What causes a nation or country to fall? What would the effects of a plague be on a country? Also, would a country invade another country that was in the middle of a plague? And how do countries become buddies, so to speak? How do countries establish trade with each other? How does a country learn the other nations' language? How do countries react to
foreigners who cants speak their tongue?

[Azélor]

I feel that your questions are a bit overwhelming. I mean that they could generate lengthy discussions and this forum is more about cartography anyway, even if we sometimes enlarge the scope to some aspect of worldbuilding.

You could either try here:
http://www.thecbg.org/

or here:
http://worldbuilding.stackexchange.com/

Try to be specific (and clear) in you questions and avoid asking several at a time unless they are closely related.