Why do we call Japan 'Japan' if they call their country 'Nihon'? 중국, 마르코 폴로, 말레이시아, 포르투갈의 영향으로 만들어진 단어 Japan - 세계화가 진행되기 훨씬 이전부터 언어는 세계화의 경로를 밟았다 - Korea (고려)도 마찬가지

 

The English word for Japan came to the West from early trade routes. The early Mandarin Chinese or possibly Wu Chinese word for Japan was recorded by Marco Polo as Cipangu. The modern Shanghainese (a dialect of the Wu Chinese language (呉語) or topolect) pronunciation of characters 日本 (Japan) is still Zeppen. The old Malay word for Japan, Jepang (modern spelling Jepun, although Indonesian has retained the older spelling), was borrowed from a Chinese language, and this Malay word was encountered by Portuguese traders in Malacca in the 16th century. It is thought the Portuguese traders were the first to bring the word to Europe. It was first recorded in English in 1577 spelled Giapan.

From this Wikipedia page

EDIT: For everyone saying this is ELI5 not Wikipedia, here's a TL;DR:

The English word for Japan came from early trade routes. The word stems from a Malay word Jepang, which was brought over to the UK by some lovely Portuguese folks. It was first seen in English in 1577 as Giapan.