What’s below is intended as a very short checklist to make ircing in UTF-8 possible using PuTTY 0.60, screen 4.00 and irssi 0.8.11. The settings probably apply to later versions as well.
Select “UTF-8” in the pull-down list at Windows → Translation in the configuration dialog.
Set the environment variable LC_ALL to a locale name like en_US.UTF-8, fi_FI.UTF-8 or something similar. If your shell is bash, sh, zsh or ksh, the command is:
export LC_ALL=en_US.UTF-8
For csh and tcsh, the syntax is:
setenv LC_ALL en_US.UTF-8
You can see the available locales with command locale -a.
When starting or reattaching screen, use option -U. For example:
screen -U
or
screen -rU
After starting irssi, invoke the command:
/set term_charset UTF-8