Provide it with a font that supports them. Although I never intended it to be used in languages different than russian and english (historical reasons) and never got to check if it would support any other. However it _should_ be possible since engine uses utf-8 encoding. All you need is a font.
i tested it with a font that supports accented characters and it somehow turns them into cyrillic characters. as you said, that shouldn't be happening if your text is in utf-8. i think it may be a bug.
i would recommend at some point down the road allowing menu.script to reference an image for buttons instead of only text.