Some of its parts (drivers and the surrounding areas) are, some parts (most of the GUI) are not. So, it basically requires having the "correct" language (for non-Unicode programs) set, if everything should work 100%.
Of course, that's a problem if you work with multiple languages (with different encodings), or if there are multiple users on the computer, each using a different language.
Seing the code for the subject check, I can understand the problem... however, it's not clear what to do about it. The subject check may have some sense a couple of years ago, but it's mostly useless today - so the best thing to do is to remove this check/option alltogether, which will most likely happen in avast! 5.0 (even though it will be fully Unicode).
You can safely disable the "Subject structure check" on the customization page of e-mail heuristics, you really won't miss anything.