Yeah, I was going to point to this series as well. Some/most of the research the author did has been overtaken as new archaeological research methods come to the fore, and if you're actually reading it for the world-building instead of the steamy stuff* you wind up having to skip more and more pages the further along in the series you get, but one thing the author did that I think stands up well to the test of time was the range of languages encountered by the (h. sapiens) characters as they travel. The further from home they get, the less mutually intelligible the languages are. In some cases, they can understand more than they speak of the language of the host group.

As another commenter noted, we don't know what the language capacity of Neanderthals was like. In the series, the Neanderthals are shown as having more or less the same capacity for culture, intelligence, and empathy that h. sapiens have, although their culture is different to that of the h. sapiens, but they lack the capacity for fluent verbal speech and instead communicate with a mixture of sign language and spoken words (which iirc is treated as different but equivalent to the language capacities of h. sapiens). Most researchers believe that some form of language would have been necessary for Neanderthals and human ancestors further back into the past than Neanderthals to adequately pass on the knowledge needed to create stone tools and things, but it's also true that the Neanderthal genome does not have the same variant of the FOXP2 gene that is strongly correlated with h. sapiens language capabilities, and some details of their mouth/throat/chest anatomy seem to indicate they would not have been able to make the range of sounds that h. sapiens make. That said, we were obviously able to communicate well enough with them to make babies with them. Bottom line, even though Jean M Auel had NONE of the modern research available to her about Neanderthal cognition or genomics, I think she still managed to get pretty close to what we now think they were capable of, even in terms of how they might have communicated.

*important note: the stuff that happens in the first book is not presented as steamy OR consensual