Button labels can currently only be configured to use a single font.
The easiest workaround might be to make your button transparent (Widgets -> Show Transparent) and either draw the desired diacritics underneath the button on the card background or position a field underneath the button that uses rich text.
I know this is a very inconvenient approach for representing non-English characters, and I'm working on a better way to address this limitation that will hopefully be ready for release in the near future.