Hi Chouby !
I solved problem.
it was the language's name.
when I added Korean to polylang its name was '한국어' which its own letter.
WP doesn't recognise it.
so I changed language name '한국어' to 'Korean'
and now it works perfectly.
I tried Japanese as well. 日本語.
if you want to use Japanese, you need to change its name when you install(add) language.
Thank you.