unibyte-display-via-language-environment

unibyte-display-via-language-environment is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil means display unibyte text according to language environment.
Specifically, this means that raw bytes in the range 160-255 decimal
are displayed by converting them to the equivalent multibyte characters
according to the current language environment. As a result, they are
displayed according to the current fontset.

Note that this variable affects only how these bytes are displayed,
but does not change the fact they are interpreted as raw bytes.

You can customize this variable.