It works fine so long as the users are using Unicode. It's possible they aren't, though. In such a case, you will need to tell them to start using Unicode, for the sake of newer clients.
mIRC 7.x is intentionally only compatible with Unicode and not the old Windows regional codepages of the past. The old regional charsets were complex to support and added a lot of code to mIRC that made it a lot harder to maintain. The new Unicode codebase does not mean it does not support international characters-- in fact, quite the opposite. Unicode has 65535 (technically more) characters to ASCII's 256. It supports just about every regional script out there without having to change to a specific region. Therefore when people are using Unicode, they can have conversations in French, German, Japanese, Russian and English at the same time, in the same window, without changing their locale. Once other clients (and users) start adopting Unicode, IRC will be a much better place. mIRC is already taking steps to make this possible, and most other IRC clients already support Unicode out of the box, so the transition might take a little bit of time, but it should go smoothly once mIRC is out of beta.
It's actually more likely that the clients that show gibberish are using an older version of mIRC without Unicode enabled. You should tell those users to enable Unicode (mIRC 6.17+ supports Unicode) and then you will be able to see what they write too.
