The Cyrillic U8g2 fonts work fine on the on 1.54 inch E-Ink display, both for Russian and Ukrainian languages. The only issue is that the largest font u8g2_font_10x20_t_cyrillic is still too small.
There is no such problem for French and German fonts with special characters. I included the photo of my E-Ink display and the code in my previous post on this subject:
u8g2_font_10x20_t_cyrillic is bigger than u8g2_font_cu12_t_cyrillic or u8g2_font_unifont_t_cyrillic
The Adafruit_GFX library only works with letters from 0-255 i.e. 8-bit. Unifont letters require 16-bit. And language letter groups are not always adjacent.
16-bit Unifont letter rendering might be a useful feature for Adafruit_GFX but would be a nightmare for Adafruit support.
U8g2 can handle almost anything but was designed for smallish displays. So you would not expect to use very large fonts on a 128x64 display.
Yes, it would be handy if Oliver added one large Unifont font to u8g2lib. It would make little difference to overall library size or build speed.
Oliver's U8g2_for_Adafruit_GFX library seems the best way to deal with non-ascii fonts.
I solved the problem. I found at the GIT site of the U8g2 project GitHub - olikraus/u8g2: U8glib library for monochrome displays, version 2 the following font u8g2_font_inr24_t_cyrillic in the file u8g2_fonts.c (the complete font code did not fit into the post, I provide it here only as an example).
For some reason it is not in the downloaded library yet. Then I pasted it in my sketch and voilà, - I can use the font u8g2_font_inr24_t_cyrillic all right. Thank you for your assistance!
I think soon this font will be in the updated library.
I have created version 1.7.0 of "U8g2 for Adafruit GFX". It should be available within the Arduino IDE in the next days (depends on the Arduino Library scanner).