https://bugs.winehq.org/show_bug.cgi?id=50676
--- Comment #3 from lunarlambda@gmail.com --- I also found that this seems to be specific to dialogs and strings created in a resource script.
I wrote a simple program that calls MessageBoxA with some UTF-8 encoded japanese text, where the text turned out garbled even when running in ja_JP.utf8. Only using wide strings and MessageBoxW could I actually get the string to display correctly.