It's definitely an encoding issue. The new mIRC uses Unicode as it's the best encoding available (it handles all languages instead of just certain ones). As long as the other people are not still using code pages, they should have no trouble displaying the text correctly. mIRC has supported displaying Unicode for a long time.
Whether that is a code page issue or they (or you) are using a script that is doing some encoding or decoding of its own, it's hard to say from that little bit.
In short, the newest mIRC can send and display all Unicode fine without errors. This includes all languages. If someone can't be bothered to upgrade, then they are the ones with the problem and not you. They either need to upgrade or accept that not everything will display correctly. If you feel that you are the one who has to bend over backwards to make it display for them, then you can always use mIRC 6.35 instead. But it's really their job to upgrade.
