Does it explicitly state that he is speaking basic?
Because there are plenty of times in the EU when they've just written what aliens say even though they're speaking a different language.
He talks to and with everybody (as do other Trandoshans in both books and comics, by the way, I checked), so either he speaks Basic, or Dosh is surprisingly widespread in the galaxy.
So, Trandoshans are able to speak Basic in WEG's RPG and are shown to speak Basic in older EU material. TCW/Rebels did not change anything in that regard, they are actually faithful to the existing wider canon.
Yeah, but, greedo speaks huttese with Han, and he doesn't bat an eye.
Nobody ever asks what chewie is saying (that I can recall, anyway), everyone just understands him.
The only one I ever see having an actual translator needed is Jabba (and I suspect that's mainly for show) and the Ewoks.
The Kubaz in Ep IV doesn't speak basic when he's talking to the stormtroopers, and they still understand him.
Tons of the Cantina patrons are speaking various languages and there seems to be no trouble understanding them from anyone in the cantina (other than luke).
People seem to understand R2-D2 all the time as well.
It seems that languages are simply "understood" most of the time.
So unless it explicitly states that Bossk is speaking basic in those books, I'd think it's more for the ease of reading than anything else. (especially since he speeks dosh in Ep V)
Han understands Chewie. C3PO understands R2, Luke understands R2 when they are in the starfighter and can read what he says.