[ijzeren_jan]

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.