You do realize that Unicode does not mean international characters stop working, right?
In fact, the reason mIRC adopted unicode was for the exact opposite reason-- to improve support for multiple languages. Ä Ö Ü ć, and all characters continue to work in 7.x. The only difference is that when you type them, they are sent as UTF-8, pretty much a worldwide standard encoding to represent your text. 6.35 can display Unicode, so they should be able to see what you wrote (though it might require enable UTF-8 support).
If you tell your friends to also enable UTF-8 messaging, you will see what they are writing too.