Ming from Agents of Speech explains that language delay in children comes from trouble grasping grammar and how language works, not from the language itself. A child learning a second language can use rules from their first language, but delayed kids struggle to build those rule systems at all.
You have to understand the reason why children are speech and language delayed. What is that? When children acquire language, they have a problem acquiring the language. It's not about learning a specific language, so to speak. It's very different from a first language learner and a second language learner. Let's say I'm learning a different language. I will reference to my first language and I will think about the grammar. And I already know what's the past tense, what's the article, what's a noun, what's a verb, whatever. I know where the places are supposed to be from. For instance, if I'm learning Indonesian, I realize that the adjective comes behind the noun instead of in front, like Chinese and English. Okay? So because I have that knowledge, I can think about it in that way and I will know, oh, the grammar is like this in this language. Oh, I get it now. And I can make rules up. So language at the end of the day is a set of messages that have rules defining it. So if a child has a problem acquiring those rules, then we talk. That's basically speech and language delay.
