I also found that Ainu, Japanese minority language, is
missing.
unfortunate.
Not so much. As I wrote, as far as plain text display (or even somewhat
rich display) concerns, it does not matter.
Is Ainu supported by JIS ? By 10646.x ?
Ainu does not have its own writing system and usually uses Japanese
writing system. So, unless you are trying to something more than
plain text display such as spell checking, it does not matter.
Thus, general purpose header:
Content-Language:
can not be introduced lightly.
(im not even sure Ainu exists in written form, tho that is most likely due to
my
ignorance).
Your knowledge is quite correct.
Masataka Ohta