Extended character support in AGI is complicated. Getting the kind of results you are hoping for is doable, but is extremely difficult and I mean
EXTREMELY DIFFICULT.
The WinAGI help file provides detailed information that explains the limitations of AGI in handling fonts. But to summarize, AGI can display the extended characters when in text mode without any problems, but in the graphics mode, extended characters will only display properly if the background color is black. Also, there is no way to edit the font that AGI uses - it comes from the built in MSDOS font.
That means you can't use extended fonts for the 'regular' messages that get printed in black text on a white background with the traditional red border.
Unfortunately, this limitation can't be modified using 'normal' AGI programming techniques. But there are some advanced programming techniques you can use that will modify the AGI program as it is running to get extended characters to work. I described the technique in this post:
https://sciprogramming.com/community/index.php?topic=1881.0If you only use interpreter version 2.917, then the solution in that post will work for you. But if your game uses any other version, it won't. You could create a new logic that duplicates the technique, but you would need to understand assembly language and know how to disassemble the original interpreter.
Sorry I couldn't be of more help, but that's just the way AGI works. It was not designed to be supportive of extended characters.