Because Textpattern and the browsers caused problems when I wanted to try TXP on an international domain (that thing with umlauts), I used Wordpress instead. I already have quite a bit of experience with it. However, not so far with UTF-8 character set and not with international domains either.
Result: the same error as in the TXP Admin - the Apache header is not being set. Pretty annoying, since browsers nowadays - correctly - prefer the Apache header over the meta tag. And when you want to change the URL in the options from the automatically filled technical address (this xn-- stuff) to the correct international address (the one with umlauts), there are problems. The server does a redirect that doesn't work. If you correct that, the whole thing still doesn't work - it simply doesn't get saved.
By the way, Wordpress works with Opera - the only browser that handles international domains correctly - only very limitedly. Both the layout causes issues and the problem described above is also there in Opera.
Somehow I have the feeling that you shouldn't run a CMS on international domains at all, but rather use these international domains only as a redirector for the actual main domain. Because not much else works reliably with these annoying things...