IIRC doesn't he kind of hiss in ESB? Hopefully they'll have his language be a derivation on that and then use subtitles.
The fact that the show is on Cartoon Network and for kids makes me think if he has any quantity of lines he'll speak basic so little 5 year olds don't have to read it. We can all know he doesn't really speak basic and that he's hissing but it was translated for the kiddies. I think I'll still like Bossk and he won't be ruined.