You can install additional languages using a link in the language selection dialog.
However, avast! 4 GUI is Ansi-based. So, while you can switch the languages are you say, it's important that the regional setting of your Windows (in particular, the "Language for non-Unicode programs" option) matches the selected avast! language, otherwise the GUI won't display correctly.
Well... since Malay uses only English characters, as far as I can say... they might work all if you have the "Language for non-Unicode programs" set to Simplified Chinese.