I also want to make it accessible to everyone, in any latin-based language.
Here is an example of an Icelandic football player whose name contains exclusive Icelandic letterforms:
Gylfi Þór Sigurðsson
Therefore, I think it would be worth my time to make sure these unusual letterforms are created to prevent my font falling short.
Here is my development of the (eth) and the (thorn)
Pretty simple composition of the 'thorn' using the existing line weights and curves of the typeface. I made the extension slightly shorter to make it look more consistent.
The hardest part of the 'eth' was to get a smooth curve around the circle and up into the top left corner. Once I had done this, I experimented with the horizontal line placement and angle before finalising one that I was happy with (bottom right).
The 'sharp S' character native to the German language was difficult as I didn't want it confused with a B. I curved the otherwise sharp top-left corner of the character, and made sure the middle bar and the bottom component didn't touch, but also didn't seem too distant.