Why “LANG=C”? (not D or E or F)

Question

In order to turn off localisation features one should set LANG environment variable to "C".

Why "C"? Where it came from?

Answer 1

In the C programming language, the locale name C “specifies the minimal environment for C translation” (C99 §7.11.1.1; the principle has been the same since at least the 1980s). As most operating systems are written in C, especially the unix-inspired ones where locales are set through the LANG and LC_xxx environment variables, C ends up being the name of a “safe” locale everywhere.

POSIX specifies that both C and POSIX must be valid locale names, with the same neutral settings.