A couple points:
...define the point at which the child learns language.
Certainly, learning language is a process, with "software","hardware", and "training" requirements. An individual's mastery must develop via "training input", but, the "ability" must be present AT FIRST. The child must already have in place the ability to receive and process input.
Do you believe small changes accumulate to become differences of kind?
This seems unrelated to the language development issue. The accumulating "changes" in language learning do not change the child's physical being. No, I don't believe this example or the ring species examples (which started my mental meanderings) show evidence of accumulating small changes becoming "differences of kind".
My analogy of evolution, as developed the past few days:
Uncle Joe who lives in California gets dropped off in front of your house on the east coast. You see the car pull up and he gets out with a suitcase that has a collection of stickers from different states, some of which have dates on them. You develop a travelogue of his journey from California based on the sticker collection.
You may get the sequence of stops along the journey correct, but, it is highly unlikely, since you can't really be sure if he travelled all the way by car or by which route or if he flew across country. (The inability to determine his mode of transportation makes the travelogue practically impossible.) The stickers may have nothing to do with this particular journey.
...your inability to comprehend evolution doesn't change reality
Yes, reality is unchanged by my inabilities - but, who can fault my inabilities? If anyone says that they comprehend evolution, with its myriad of possible travelogues, I'd look at them cynically.
First, there is no logical difference between hardware and software, either in computers or in humans (or animals). In animal brains, learning is most definitely a physical change. You can see it under a suitable microscope. Learning changes the physical connections between neurons.
Second, language does require a suitable brain structure, but then that has no relevance to the discussion. Evolution also has to work with what is available for modifiction. Humans are not going to evolve feathers. If they do, get back to me and I will admit biology doesn't really know anything.
>>The stickers may have nothing to do with this particular journey.<<
True, and the story has no relevance. If you wish to believe that some omnipotent trickster diddles in DNA, placing false markers at will, that is cerrtainly a possibility. But it's a fairy tail with no Scientific merit or Biblical authority. Scientology makes as much sense.
In a universe in which DNA has no actual lineage, and false markers are placed by space aliens or deities, you cannot have science, because anything can happen without cuse or trace.