Orcas have been teaching each other to sink boats, from what I was reading. They don't speak, but they do communicate.

> That would mean a single individual not able to gain from previous generations' knowledge.

Surely not. Skills can be passed on by imitation and observation.

That being said, I’m certain they had language too.

Sign language, whistles and probably spoken language too since they probably co-develop.

Neanderthals lived in small groups in sub-zero temps. They had to co-ordinate or they'd die. So I'd say, yeah, they could indeed speak and communicate in complex manners, much like us, or maybe even better.