it's not really memorization, children don't learn languages by sitting there desperately burning every word into their memory, they hear it repeated over and over and the brain just passively soaks it up and eventually starts making sense of it.

And i'm pretty sure this is why people who learn to speak languages in school are generally terrible at them, it's shoved down their throat and the brain is given no time to process understanding the language before trying to speak it.
I've found that my ability to speak languages i learnt in school has become significantly better a couple years after the fact, presumably from my brain having had time to process the information.