What's wrong in reading the complete documentation, the examples and then the source code of said programming language? Debugging through also helps sometimes on tricky problems.
That's usually the easiest way to learn a new language.
That should be done as well, the flashcards are just a supplement. I find there's often a weird transition where if I'm just reading docs as if it were pleasure reading, I'll be nodding my head, "yeah, that makes sense", "yeah, that's easy", "yeah, I understand", then all of a sudden, "wait a minute, I'm totally lost and don't remember a thing from the last five pages!"
Also, it's easy to get the bird's eye view of a language (say, React, for example), but when you're down in the trenches, you need to KNOW about the pedantic little details (like "class" vs. "className", "for" vs. "htmlFor", minutiae of React lifecycle, etc.)
That's usually the easiest way to learn a new language.