WFRP 3rd is the first Warhammer rpg I play, therefore I know next to nothing about the warhammer setting. After reading the core rules, it is still unclear to me how various races communicate with each other.

I assume Humans, Elves and Dwarves can all speak a 'common' language, else there would be no mix-groups of adventures.

But can adventurers communicate with Beastmen?

In the core books, it is written that Orcs sound like thugs and goblins like thugs after inhaling helium. Does this imply that they can communicate with the PCs?

This info on languages from Warhammer online

http://warhammeronline.wikia.com/wiki/Language

..is still pretty accurate for the RPG, and gives you some of the common different languages and their names and areas of use.

To answer your question, it is simpler to assume all PCs and NPCs can normally speak Reikspeil and therefore have a common language in which to communicate, but that a dwarf PC could also speak to a dwarf NPC in Khazalid, if needs be.

Training in the education skill could be used to determine how likely a PC is to know a language that isn't their mother tongue (other than Reikspeil, which we assume everyone can speak to some degree or another, for simplicity).

To answer your question about Beastmen, although the languages article I have referenced says Beastmen generally don't speak Reikspeil, it has been known that some human mutants that have turned eventually into Beastmen do retain some of their vocal cords and so can still communicate in Reikspeil, given it be in a guttural and twisted manner.

Having Beastmen, Orcs, goblins or whatever communicate with the PCs (even if it is just to insult them) certainly wouldn't break any of the Warhammer fluff to any great degree, if that is what you are concerned about, so go with what-ever feels right or you need for a particular encounter.

Having said that, you can't beat playing charades with a snotling as a way of comminucating!!

Beastmen (those that can speak) speak: Dark Tongue/Black Speech. Their "rough" way of speaking can be called "Beast Tongue". But I would say it is very much possible that few Beastmen may actually learn to speak common languages (Reikspiel) in also, and this way speak with characters. Same goes for the Greenskins.

Here is little longer list of languages I have collected (list features also WFRP2 rules...): Kalevalahammer - Rule Expansions