system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why don't we call countries by the names they call themselves?

For example, Germany is Deutschland.  Why do we not refer to it as such?
gpt
This is an awesome question...I have ALWAYS been curious about this (specifically in the example you have provided).

Looking at it again, for the first time, it appears to be a situation where the name "Germany" developed for that region LONG before Deutschland became a unified country and they started referring to themselves thusly. The Romans were calling the land area "Germania" when they first visited because that was the tribe that they encountered...the Spaniards were calling it "Alemania" because of the tribe closest to their border. 

Looking at the United States, we are called many things that don't really sound like United States:

* Vereinigte Staaten von Amerika (German)
* Sahalat (Laos)
* Estados Unidos de América

With Japan, it is a bastardization of the original name Cipangu...it went through several iterations as it traveled around the world by ship that eventually resulted in "Japan" in English.

(Thanks to MentalFloss.com for the additional information).