default-file-name-coding-system is a variable defined in `C source code'.
Its value is

Default coding system for encoding file names.
This variable is used only when `file-name-coding-system' is nil.

This variable is set/changed by the command `set-language-environment'.
User should not set this variable manually,
instead use `file-name-coding-system' to get a constant encoding
of file names regardless of the current language environment.

On MS-Windows, the value of this variable is largely ignored if
`w32-unicode-filenames' (which see) is non-nil. Emacs on Windows
behaves as if file names were encoded in `utf-8'.