This is actually a good example of users of the language adapting it to their needs. "didn't could speak" is not technically correct, but I'm not condemning it. You hear this in some Southern speakers especially, e.g.
"We used to would go to that park"
"That car needs fixed"
"I might could use those verbs correctly.
The more complicated verb tenses in English are pretty damn hard for people, even native speakers. So they adapt.
For the record, the "correct" English would be:
"I couldn't necessarily speak it well" or
"I didn't necessarily learn to speak it well"