The English language uses a blended NG ("ing") the vast majority of the time. It would, hence, follow that the name Dengar should be pronounced "Deng-ar" as opposed to "Den-gar".
I have never, ever in 30 plus years of Star Wars conversations heard anyone pronounce it like that.
My question is this: does anyone here pronounce it "Deng-ar"? How about "Deng-gar" (more like Eng-gland)? Anyone? I highly doubt it.