Alright, so I've always heard Goblin was a lot like Common. But when you listen to the 'goblin' voices, they all have un-Common words thrown in there. So is it kinda like extremely broken common (Kinda like how Patois sounds like English)? Or is it just the sentence structure and grammar? Or maybe common with a buncha extra words thrown in? Or what?

Goblins speak their own language, but largely have a very basic, broken grasp of Common as well.

Undercommon, actually. But it doesn't really matter, as everyone can understand any language they desire in the Underdark. Except the Svirfneblin runes in Dunwarren. ;-)

Have 30+lore, then you can understand any language, :P