A word to word translation would be "what is it that is so funny"? So the main clause is" vad är det ", and" som" leads a subordinate clause "som är så roligt", which describes "det" in the main clause.
Yes. However if someone is laughing for no apparent reason you would ask "Vad är det som är så roligt?" or possible "Vad är så roligt att du inte kan sluta skratta?" but not just "Vad är så roligt?"
Why wouldn't you say that? Too formal? So short that people night not notice? Or …?
Why does it provide both 'fun' and 'funny' as translations for 'roligt'? They are not synonyms at all
They're not always synonyms in English, this is true, but as I take it 'roligt' is an interchangeable word in the Swedish language for both 'fun' and 'funny'.
Why isn't "What is that which is so funny?" Correct? I know sounds a bit unnatural but is that the only reason?
I don't think any English speaker would say it that way, so that is probably the only reason.
Im a native spanish speaker and translating it that way makes much sense haha, sometimes swedish is really similsr to Spanish.