I’ve noticed in some parts of Wales, people use shortened forms of words or phrases
Example:
Does Dim (there is no/not) is often shortened to ‘sdim’ in colloquial talk
Anyone know any other ones?
There are lots and as you said, they vary across regions.
'mond is another common one (short for ‘dim ond’ - only), as is leaving off the ‘wyt’ when saying “wyt ti” e.g. “ti’n iawn?” instead of “wyt ti’n iawn?”.
Then you’ll hear words where first syllables often get dropped (which is a nuisance when finding them in a dictionary!) such as 'sgwennu (ysgrifennu - to write), 'sgwyddau (ysgwyddau - shoulders), 'gliniau (pengliniau - knees), even paned is an abbreviation (short for cwpaned!).
But then again, shortening like this is not peculiar to Welsh - after all, how often do we say don’t / won’t / gonna / dunno / “you ok?” etc in English! 
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