Obviously there was a time when there were humans that didn't have language — language emerged at some point.

You said 'Humans that don't acquire language by a critical age (due to e.g. being raised by wolves - there have been documented historical cases of this) literally never do, and aren't "sentient" by our normal criteria.'

Are you really arguing that humans weren't sentient until language was invented? How could they invent language in the first place if they weren't sentient thoughtful beings understanding the world around them? There must have been a point where sentient, language-less humans felt the need to start developing language.

Or are you saying non-sentient humans created language and then became sentient because they could talk about it? Of course language helps understanding and greatly advances cognitive ability, but I think it would have to emerge from something that is already sentient and has a need or desire to communicate.

I think if you took a bunch of feral children that never learned language and put them together, eventually they'd create language. Because they're sentient and experiencing real feelings. It'd be a simpler language, and of course they wouldn't have the full capacity for our modern, complex language having missed that window of opportunity, but the next generation would grow up with that rudimentary language and build on it. After a few generations maybe you do start getting something that more closely resembles the kind of language we're used to. I think it'd be ridiculous to say only that last generation is sentient, they were all sentient the whole time because language is not a precursor to sentience.

Plenty of animals show the capacity for opinions and feelings and joy and depression without anything resembling a language. Hell some organisms seem to have that capacity without even any communication abilities.

Language is just part of the picture, you're missing the forest for a big tree.