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I'm not sure if I'm just imagining things or if I remembered things wrong,
but is there some kind of colloquial dialect where sentences end in "-jai?"
You are not imagining things. (The tougher) Kansai male speakers do say 「～～～じゃい!」 in colloquial speech. Only the 「い」part is a particle -- an emphatic sentence or phrase ender. And じゃ ＝ だ.
The Standard counterpart would be だよ or だい.
The Kanto countepart would be だい or でぇ.
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