Don't worry, those letters are not part of the standard orthography of Interslavic. Although some people insist on using them, they are merely meant for educational purposes, and to make a direct link to Old Church Slavonic. The only non-ASCII letters in the Latin alphabet that are mandatory are Č Š Ž and Ě (although in Interslavic nothing is mandatory). And of course, Interslavic can be written in Cyrillic as well, as is done by many users.

It seems like they support both Latin and Cyrillic transliterations.