To set global decode and encode charsets, use "/setp" command. For example:
/setp charset.global.decode = ISO-8859-15 /setp charset.global.encode = ISO-8859-15
If global decode charset is not set (for example during first load of Charset plugin), it will be automatically set to terminal charset (if it's different from UTF-8), or by default to "ISO-8859-1". Default encode value is empty, so it sends by default with internal charset (UTF-8).
To set server charset, use "/charset" command on server buffer. If you give only charset, then it will set "decode" and "encode" values. For example:
/charset ISO-8859-15
It's equivalent to:
/charset decode ISO-8859-15 /charset encode ISO-8859-15
To set channel (or private) charset, use same commands as server, but on channel (or private) buffer.