The word originates from the following biblical tale:
And it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay; Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand.Slain for not being able to pronounce a word properly; pronounciation betrays our origins. In the UK it not only tells you where you come from but also which social class you belong to. You can wear the right clothes and drink the right wines but as soon as you open your gob, a stray flat a will give the game away.
Judges 12:4--6, KJV
I always found it a bit implausible when, in countless war films, British officers disguised as locals would be challenged by German border patrols whilst trying to escape. They would reply with what must've been perfect German pronounciation, for the guards never ever cottoned on.
In Denmark an unofficial test to establish your Danishness is to pronounce rødgrød med fløde (a red fruit dessert with cream). To a non Danish ear it sounds like someone trying to talk with a mouthful of hot potato. In British English the neutral vowel, and how well you throw it away, is a dead giveaway of the impostor.
As for Wales, many would argue that you could pick any Welsh word at random as all are incomprehensible to the non Welsh ear. Judging by the awful attempts BBC newsreaders make at pronouncing the simplest of Welsh place names, I'm inclined to agree. They take immense pride in navigating their tongues through a list of Swahili villages but ask them to pronounce Llanelli and they're lost.
Newsreaders must go to bed at night praying that nothing interesting will ever happen in Machynlleth or Dolgellau.