By reworking your code once and making your code support Unicode, you get the world's scripts in one stroke. Version 2.0 is identical to the ISO 10646 standard of the International Standards Organization, and is known as KSC 5700 by the Korean Standardization Body and JIS X0221 by the Japanese Industry Standards Body. Not everyone is using Unicode yet, but there is a definite push to use Unicode in the future. It would not be farfetched to say that any developer who isn't at least planning to use Unicode into future versions of international software is going to have a hard time being competitive internationally. For anyone wishing to sell software to overseas markets, Unicode is one internationalization strategy that should be a mandatory consideration.
Most Asian languages are double-byte to begin with, so there is no change there.
For applications, text is usually a very small percentage of the code, and any increase is usually insignificant. This question is more significant when considering large databases. However with the advent of XML which uses Unicode, there has been a move to bring Unicode-support to Perl. To then produce the same software in a different language, translation of the menus, help files, and documentation by humans are, of course, necessary, but engineering changes can be kept to a minimum. In this way, Unicode-enabling an application is one step towards shortening the software internationalization process.