Here's where I am coming from: there have been a number of experiments to teach language to other species, but there is always a problem in trying to figure out to what extent they 'get' language - For example, there is the case of the chimpanzee Washoe signing "water" and "bird" on first seeing a swan - was it, as some people contended, inventing a new phrase for picking out swans (or even aquatic birds in general), or was it merely making the signs for two different things in the scene before it? [1]
One thing that has not been seen (as far as I know) is two or more of these animal subjects routinely having meaningful conversations among themselves. This would be a much richer source of data, and I do not think it would leave much doubt that they 'got' language to a very significant degree.
[1] https://www.nybooks.com/articles/2011/11/24/can-chimps-conve...