Tthe forum will basically function, provided that $boardurl is set up properly. (And, I suppose, the column in the boards table might need to be longer to hold the domain name too) I don't recall there being any checks made by the installer as to the validity of $boardurl, merely that it isn't empty, because the default value is usually right.
However, the url bbcode will not auto link internationalised domains, though it should continue to link Punycode domains (since the whole point is that they're Latinised to use the same characters in the standard character set that domains always used to have to be)