Well, the difference is the fact that everyone agrees that they’re different languages. It’s not like comparing English to Scots (is it just a weird dialect of English? Or is it distinct enough to be its own language?), or comparing all the different dialects/languages/[idfk anymore] of Serbo-Croatian to each other, especially if they’re all written in the Latin script (in Serbia, you have the option of using the Cyrillic script, but not everyone does it anyway).
Well, the difference is the fact that everyone agrees that they’re different languages. It’s not like comparing English to Scots (is it just a weird dialect of English? Or is it distinct enough to be its own language?), or comparing all the different dialects/languages/[idfk anymore] of Serbo-Croatian to each other, especially if they’re all written in the Latin script (in Serbia, you have the option of using the Cyrillic script, but not everyone does it anyway).