What century was that Diocletian? Some Serbs may have considered Macedonia a part of Serbia (just as Greece considers Macedonia a part of Greece and Bulgaria consider it a part Bulgaria) but my in-laws are Macedonian and they have never had any trouble from Serbs for speaking the Macedonian language. Outlawed? The only place where there has been “problems” for using the Macedonian language is in Greece. I’m sorry but I think you should stick to your info about Croatia and leave Macedonia out of this issue. As for Macedonians today, they have absolutely no problem with Serbs. Their concerns are the Albanians!
The first half of the 20th century. I’m sorry if you were ignorant of this fact.