I'm not really sure how this is achieved, but there are a lot of fonts that seem to be compatible both with Windows-1252 encoding and with Unicode. Take for example this one:
https://www.dafont.com/press-start.fontIf your game is in ASCII/ANSI (the way it is in the beta by default) and you import this font to slot 1, for example, "extended" latin characters will display properly (Spanish accents like á or Á, or German umlauts like ö or Ü).
Now switch your game to Unicode, and these characters will be garbled. But if you import the same font to a different slot (e.g., font 2) and reassign SpeechFont and NormalFont to this one, the characters will once again display properly.
(Note: import size is 8, and for some reason, some fonts like this one need to have the TTF font adjustment property set to "Resize ascender to nominal font height", otherwise they will be cut off at the top.)
This seems to be quite random, since some of the fonts in that page are only ANSI compatible, some are only Unicode, and others can even tolerate the conversion from ASCII/ANSI to Unicode without issues (setting the engine back to ASCII again will garble everything, though). But it might be worth checking the
"bitmap" section of the page for fonts that are unicode compatible, a reasonable size for low-res pixel games (8 to 10px) and GPL / public domain or have a "100% free" license.