In German the capital letter ẞ (Eszett) was officially recognized as the uppercase variant of the ß letter some months ago.
The translation of the previously mentioned string - "BEIẞ BEIẞ BEIẞ" - actually requires that letter but it looks weird in Factorio's font.

I don't know if it is possible to make the ẞ not look out of place, but as far as I can see from the image, the letter itself is working.