For Interscandinavian, I'm lucky. My father had his roots in Skåne, so I had from an early age a certain feeling for Danish. I have lived in the Göteborg area for most of my life, so Norway is not much further away than 'just around the corner'. Moreover, I inherited some genes useful for languages from especialy my maternal grandpa and my paternal grandma.
I translate professionally from Danish and Norwegian, among others, into Swedish. Those two normally come quite easily to me, but before the advent of the Internet, when I encountered a problem, there was a HUGE PROBLEM. For most cases, my dictionaries for other languages helped, but it's not easy to find good and current volumes for Danish and Norwegian.
Anyway, most Danish and Norwegian dialects are more comprehensible to me (and vice versa) than for example the supposedly "Swedish dialect" Älvdalsmål. And count me out when it comes to our officially recognized minority languages, like assorted Saami languages, Meänkieli (a very Northern Swedish variety of Finnish), or Finnish itself. I understand more of Yiddish (also included in that group).
Once, in a Copenhagen café, I heard two people speaking at an adjacent table. Totally incomprehensible to me, and the language nut that I am, I concentrated to guess the language. After a while, I picked up a short word or two that sounded almost Scandinavian. Concentrating even more, I finally came to the conclusion that they spoke Icelandic, but I still understood nothing whatsoever from it.