Not the parent, but I think you might be interested in reading about South Slavic languages [1], palatalization of these [2], and development of Serbo-Croatian [3], as well as Gaj work on the alphabet [4]. It's easy for someone who operates on a daily basis to hear the difference between sounds and wrote it correctly but for other people, especially those who live in different language group (Germanic vs Slavic groups) might have difficulties. But that's rather obvious, innit?

Đ, đ and DŽ, dž letters have counterparts in Polish in dź and dż digraphs and are used to represent same harder and softer sounds.

1 - https://en.wikipedia.org/wiki/South_Slavic_languages

2 - https://en.wikipedia.org/wiki/Palatalization_(sound_change)#...

3 - https://en.wikipedia.org/wiki/Serbo-Croatian

4 - shttps://en.wikipedia.org/wiki/Gaj's_Latin_alphabet