flameboi’s answer gives us the general idea I think
There is no combination that is clearly “intelligible”. Basically you have different degrees. Even the people from the north of the US sometimes complain that they can’t understand the people from the southern states. And this is true of most countries.
Then you have bigger differences, such as Flemish and Dutch, or Czech and Slovak, which most people would consider as different languages (I’d say they’re versions of the same one).
My Polish girlfriend insists she can’t understand Czechs, but I’ve always found the two languages to be very similar, and even though there are differences in pronunciation and spelling, the roots of most words are the same. I’m sure any speaker of a Slavic language could understand another one if they tried hard enough. The same goes for Nordic languages, German or English and Dutch, Spanish and Portuguese, even Italian and French. Two of my Portuguese friends insist they have no problem understanding French, even though to me the two languages are worlds apart.
I have even found it easy to understand Romanian, based on whatever Latin roots there are in English words plus my basic grasp of Czech, Polish and Russian.
And I have found Greek (which is not related to any of them) to be helpful in all of the above.