The problem :
My system is ubuntu with an UTF-8 encoding but most of the IRC channels wher I live use ISO_8859-1 or ISO_8859-15, so what can I do ?
The possible solution :
I ran irssi in a screen: like this :
screen -U -m irssi -c mon_network_ou_server_qui_va_bien
then in irssi I ran these commands :
/set recode ON /set recode_out_default_charset ISO_8859-1 /set recode_fallback utf-8 /set recode_autodetect_utf8 ON /recode add #channel_name ISO_8859-15
The last line has to be repeated for each channel (in the channel window)