Exonym and endonym

An exonym is a name of a person, a people group, language, or place used by people of other places and languages. An endonym is a name that people of a certain place and language call themselves. For example, the endonym of Germany is "Deutschland", because that is what Germans call their country, and Germans call themselves and their language "Deutsch". On the other hand, Germany and German are its exonym because English speakers, who are not German, use those names to describe it.
This is actually very common across languages for several reasons. One reason is because the sounds for an endonym may not exist in certain languages. It would be difficult for speakers of other languages to say them, so speakers of other languages make an exonym. For example, words in Japanese cannot end in any consonant that is not an "n" sound, and for that reason, "Deutsch" is "Doitsu" in Japanese. Japanese also adds Japanese morphemes so that there is no confusion between the country, the people, and the language. The German people are "Doitsujin", in which "-jin" means people, and the name of the German language is "Doitsugo", in which "-go" means language. While the country, people, and language of Laos can all be called "Lao" in English, they are called "Raosu", "Rasoujin", and "Raosugo" in Japanese in that same order.
Another reason why a country might call another country a different name is because people from a certain country might have different ideas about those people or associate them with certain things. For example, even though Greeks call their country "Hellas", their country is known around the world as Greece because the Roman Empire named the region after "Magna" "Graecia", which were coastal areas in southern Italy that were colonized by Greeks in the early days of Rome. Germany has a very large number of exonyms, being called "Allemagne" in French, "Niemcy" in Polish, and "Saksa" in Finnish. This is because Germany has interacted with many other European countries and the Germans were a collection of tribes speaking similar languages until they unified into a country in the late 19th century. Even though the Dutch call themselves "Nederlanders" in their own language, English speakers call them Dutch (a cognate of the German word "Deutsch") because the words German and Dutch used to be interchangeable until Germany unified in the 1800s. From then on, German was used to describe people from Germany, while Dutch was used to describe people from the Netherlands.
Even though in Mandarin Chinese China is called "Zhōngguó", meaning the "Middle Kingdom" or the "Central States", most other countries calls it China. It is believed that the word China cam from the word "Qín", in which the Qin dynasty unified China. However, there are a few exceptions. In Japanese, it is called "Chūgoku", which has the same meaning as the Chinese name, except it uses the Japanese pronunciation for the Chinese characters for the name 中國 / 中国. In Korean, it is called "Jungguk" for the same reason. In Russian, China is called "Kitay".
A third reason why a country might call another place a different name is because a language can borrow a name from another language. Even though Japan is called "Nippon" or "Nihon" in Japanese, the word passed on to Europe through many different languages, all of which were very different from Japanese. Marco Polo heard about Kublai Khan sending soldiers to Cipangu, which in Chinese characters is 日本, which means "Sun's Origin". When the Portuguese arrived in Japan in the mid-16th century, who were the first Europeans to directly trade with Japan, they called the country "Japão" based on an old Malay word, which was borrowed from a southern Chinese dialect. Thus, most other countries call the country Japan. A few exceptions are Chinese, which call it "Rìběn" based on the Mandarin pronunciation of the Chinese characters, and Korean, which call it "Ilbon" based on the Korean pronunciation of the Chinese characters. Also, while many countries call European cities by their endonym, including "Roma", "Praha", "Köln", and "Napoli", the English names of many European cities are noticeably different, which are called Rome, Prague, Cologne, and Naples in the same order. These names were borrowed from French. Since French was the official language of England from the Norman invasion to King Henry V, many of the cities names came into English through French.