A spec of Email, RFC822, was defined with a hope to ensure interoperability in 1982. Since Email was grown in America, its header and body could not contains other character sets than US-ASCII.
It is, however, very inconvenient for people whose language is not English. So, despite of extension of header, many people from various countries extended RFC822 messages to contain non-English characters from their native language.
In Europe, Latin 1 started to be used that presents umlaut(accent) characters by 8 bit word. Latin 1 is sometime called ISO-8859-1.
In Japan, there are three major codes, (1) JIS code which is 7-bit 2 characters, (2) EUC code which is 8-bit 2 characters and is used in UNIX, (3) SJIS which is 8-bit 2 characters and is used in PCs. Pioneers of JUNET which is the antecedent of Japanese Internet chose a switch mechanism of ASCII and JIS with ESC sequence, so called JUNET code, for transportation.
JUNET code is sometime called ISO-2022-JP. With JUNET code, we can tell what their character sets are in addition to switch them.
The extension such as Latin 1 and JUNET code is an agreement within the region. You are compelled to use English to send a message across regions in the context of RFC822.
RFC822 is so ambiguous that we misunderstand that JUNET code can be used for header and body since it is 7 bit. Probably this is a good explanation to blow away your misunderstanding. "RFC822 defines that the syntax of header and body is 7bit and the semantics of header and body is US-ASCII". JUNET code is syntactically legal but its semantics is illegal.