system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are countries widely known as names that aren't their actual names? For example; what The West knows as Japan, is actually named Nippon/Nihon within the country itself. And there's others!

Other examples include;
Spain; named España in the country itself.
Germany is really Deutschland.
China is most commonly know as Zhongguo within the country.
And I'm sure there are plenty others.

Is it just me, or does it seem almost rude that, seemingly, these countries aren't in control of how they're named outside of their home country on a global scale? Like, if the Japanese refer to their home country as Nippon, then why doesn't the rest of the world? Why have these countries seemingly accepted the fact that they don't even control their own name outside of their borders?

Maybe I'm blowing this all out of proportion, maybe I'm just super tired and the explanation is obvious, but please help out a genuinely confused person here!
gpt
There are various reasons for this. I'll try to summarize a few.

Sometimes the country name actually means something, and so can actually be translated. For example, "United States" means, literally, states which are united, and so is translated into French as "États-Unis", into German as "Vereinigte Staaten", and Russian as "Soyedinyonnye Shtaty".

Sometimes the name of a country might change, but some languages keep using the old name. For example, the Korean for South Korea is "Daehan Minguk", which North Korea is "Chosŏn Minjujuŭi Inmin Konghwaguk": the English "Korea" refers to the name "Koryo", the name of the peninsula from the 10th to the 14th centuries.

Languages change and evolve quite naturally, and as words go from one language to another, they may undergo changes. Sometimes these are grammatical, so the "-ia" ending in Latin-based languages shows up as "-ien" in German and often disappears in German, for example. The Latin name for Spain was "Hispania": in the Spanish and French languages the "H" got left off, so we have "España" in Spanish (the "-ña" is pronounced "-nya", so that's where the "-ia" went) and "Espagne" in French (the "gn" is pronounced like Spanish "ñ"); then in German the initial "E" went missing, giving us "Spanien", and in English the ending went AWOL and the vowel was pronounced differently (part of a process of the evolution of the English language known as the Great Vowel Shift).

Sometimes the name given to a country originated in the name of a tribe living there, but when more than one tribe or nation lived in that area, different countries gave different names. The Romans called Germany "Germania", apparently the name of a tribe they thought lived there, the Germanii; the French looked over the River Rhine and the people living in that part of Germany were the Alemanni, hence the French word "Allemagne". The "Deutsch" in "Deutschland" apparently comes from a word that ultimately means "ordinary people" and was the name of the Teutonic tribe -- the process of language change means that although "Deutsch" and "Teuton" look very different, they're actually related -- and so we get names for Germany like "Duitsland" (Dutch) and "Tedesco" (Italian).

Sometimes the country's own word for their land means something like "people", while their word for one of their neighbours might mean "stranger" or something.

For example, when the Saxons invaded Britain, the Celtic tribes that they drove to the west were, to them, "wealh" = "foreigner"; hence our words "Wales" for the country and "Welsh" for the people and language. In Welsh, on the other hand, the word for the Welsh people is "Cymry" (pronounced "come-ree"), which probably means "comrades" or something similar, and the country is called "Cymru" (almost the same pronounciation as "Cymry"); meanwhile, to them, the English are "Saeson" -- "Saxons" -- and England is called "Lloegr" which probably originally meant "border", or "land beyond the border".

Occasionally, the name of a country or its people is an insult. Many Slavic languages (Polish, Russian, etc) have words for "Germany" or "German" that are based around "nemets", which described a person unable to speak intelligibly.

It's difficult to control the name of the country beyond your borders because, well, that would be interfering in other countries' languages. However, the African country formerly known to English speakers as "Ivory Coast" has asked that it be known for official and diplomatic purposes by its French name, "Côte d'Ivoire", French being the official language there. They have no authority to actually impose this on other countries except to say they won't officially recognise any other version, but it's generally been accepted.