I am surpised you didn't.:)
Your assumption is correct, all is needed is a suitable set of fonts (255 characters long for ISO encoding or whatever for UTF, couldn't understand how that works). It's also possible to replace the system font without NVDI but that is generally not clean and doesn't work with MiNT. I have created a greek language pack with instructions on how this is done that you can download from here.
http://dbug.kicks-ass.net/dbugforums/cgi-bin/yabb2/YaBB.pl?num=1259857439
The LVA accessory like the greek accessory or auto folder program probably work the same way. They replace the system font and change the way the keyboard responds to keypresses so that it accesses the characters.
I haven't found a font editor that would read one type of bitmap font and convert it to the atari fnt type. Fontkit+ could be used to convert a preexisting atari font (such as the ones I have but use the cp737 msdos encoding) if you are willing to spend the quite a few hours required for this and for the 4 font sizes but they are probably copyrighted and thus non distributable through MiNT. What I am looking at is converting some free fonts in bitmap format (bdf, psf, pcf) to the atari fnt one. Unfortunately I am too bad a coder to do this. I don't give up though :).
@Bohdan thanks for looking into it :)
Regards
Christos