From reading the Beastmen descriptions it is not clear to me if they can communicate with humans. Since they wear armor and use weapons, and have camps and a social structure, I would assume they can communicate ... but can they communicate with humans (speak common, using the DnD term)?

Yes, in a crude way.

Beastmen as I understand it, use a mix of gutural/bestial sounds, dark tounge and the local human language (where reikspiel would be the "common" tounge in the empire). So simple, straightforward phrases should not be a problem, for example "You, Die, Now! Grooaaargh!" would fit.

If a group of PC's are spying on the beastmen and are trying to hear and understand what they talk about it's basically up to the GM. There could be nothing understandable, some tidbits of information or a lot of info, whichever fits your story best.

And also in my oppinion shamans might be better at reikspiel than the average gor, ungor or even wargor. But that's just something I think is fitting.

So as an answer to your question, as much or little as you want them to be able to communicate. But they will probably never use especially advanced language.

I'd say that not all of the beastmen can. Those born as beastmen in the tribe would most likely only learn the Dark Tongue that (IIRC) is a corrupt version of what the demons speak.

However there usually are a lot of mutants and turnskins (beastmen who were born as human) in any given beastmen tribe so there should always be someone/something that could communicate with humans. The attitude of those left to woods or driven from society towards those who abandoned him (humans) could be a trouble though and increase the difficulty of the social encounter. Which is always nice!

It should be noted also that the image of beastmen as goatheaded similar looking things is a requirement of Fantasy Battle where the troops need to be distinguishive and relatively easy to produce. In WFRP beastmen have traditionally been more varied having all kinds of animal heads and mutations.

Thanks for the comments guys, will take this into account.