That said, plenty of humans do not have a voiced internal experience. The lack of language does not imply a lack of cognition. I would expect that the brain of a closely related organism, say a chimp, would have many similar experiences generated by the same stimuli. Would they experience green like I experience green? I can't even say that about a person sitting next to me, but they probably have an equivalent experience.
That said, if we had a way of communicating could we reach agreed terms? I can do that with my cat, so I would think he has an understanding of me and my behaviours along with what tends to happen when I do certain things like clap then shake my hands at the end of a treat session. He knows there are no more treats, he associates that with my hands clapping and shaking, so we communicate. Does he have a voice in his head describing it? Probably not. Does he have Meows? Again, probably not, but he would have a sense and memories of previous times.
I meant to say language in the linguistics sense, a series of abstract items which can be arranged to convey arbitrary meaning. For example, a dog barking can be a threat display, a warning, playful, sad, afraid, and so on. But can you use barking to create grammar? With grammar you could have labels for items in the world and use various barks to refer to them, make requests, ask questions, and so on. Some types of animals have warning calls that are specific to types of predators, for example an eagle call or a leopard call. Leopards require different responses than eagles so the distinction is very useful and helps others to respond. This is not quite language but is definitely a step in the right direction.
So yes, you are correct, lots of animals can communicate things to each other, but it is not the same as language like what humans have. Could we find an animal that does have language? Or something very close? Sure, but we haven't shown that yet. Maybe we should focus on giving other animals a chance to develop before we wipe them all out.
but it is not the same as language like what humans have
And it doesn't have to be. In fact this sort of thinking can limit how we learn about other animals.
Dogs don't pass the mirror test for self awareness like some corvids and a few other animals do. But dogs don't experience each other primarily through vision, scent is much more their main sense. So is that even a valid test for canid self awareness?
This is something I've always been highly skeptical of. As a somewhat experienced meditator, I'm hyper-aware of the constant flood of self-talk happening in my head, but I don't remember paying particular attention to it before I started practicing. It has always been there, but until then, I hadn't paid any special attention to it. Whenever this subject comes up with people who don't meditate, they often seem to live under the illusion that, except for intentional thoughts, their mind is more or less silent the rest of the time. I'd argue that 99.9% of people couldn't sit for 20 seconds without letting their mind wander, even if their life depended on it. Even I couldn't, despite my experience in meditation.
That's why I think that when people are asked whether they have this inner voice or not, some say no because they're not aware of it. Not having it would effectively be synonymous with being enlightened.
I have internal voice sometimes but not others. In some things my cognition is far more verbal, working through something like a monologue or conversation. Other times I may have more of a mental image of something, sometimes more in real space and sometimes completely disconnected from real space. Sometimes it is much more abstract with sensations and emotions with very little in terms of concrete metaphores.
Also I can have racing thoughts without it being language. I also have impacts on those racing thoughts from taking Ritalin (ADHD medication) and it is not just the word thoughts but also the flow of other types of cognition.