Even we native speakers sometimes have difficulty understanding some dialects; therefore the more so for second language speakers.
As to accent, having spent time in India and married an Indian, I am pretty good at Indian English, which can be very hard for many native speakers to understand. Scottish, on the other hand, sometimes defeats me. But Irish English I always understand and delight in.
As to dialectal terms, there are very many. Think back to "500 miles" by The Proclaimers. Who in the world knew what "haever" meant? Only because of that song I now know that "haever" means "talk rubbish." Another example is "godown" which means "warehouse" but is nowadays unknown outside of the subcontinent.
I have often wondered how well a Texan with a big drawl and a Scotsman with a heavy brogue would understand each other.
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