It's because mIRC primarily uses the ASCII charset. In ASCII those characters you listed aren't, strictly speaking, letters. They're actually part of the 'extended character set' which means that they don't have any specific meaning and might not look at all like an A or an O with umlauts above them in a different font.
Apparently mIRC will support Unicode in the future, which does have specific characters for those letters you've listed so when that's added it will (hopefully) work like you want.
