kermahillway:

Playing the undermine campaign had me wondering why Renzik didn’t have a voice actor. I now know why but for the time I didn’t, I started headcanoning him as being mute.

You’d be hard pressed to find another goblin who knows signing but since it’s simply in character for Gazlowe, it only makes sense for him to have learned it. That also does mean that Renzik can rip into someone without them ever knowing.