Today I was performing an emerge --update in my Gentoo box and got this error when emerging the editor nano:
*** UTF-8 support was requested, but insufficient UTF-8 support was
*** detected in your curses and/or C libraries. Please verify that your
*** slang was built with UTF-8 support or your curses was built with
*** wide character support, and that your C library was built with wide
*** character support.
The problem was due to the fact that I had added the unicode USE flag and I hadn't re-emerged ncurses.
Thus, re-emerging ncurses and then nano fixed this problem.