I agree with others here that learning a language has a lot to do with learning an ecosystem, libraries, and culture. To contribute to an existing project, it's not necessary to really know the language that intimately. I "know" a handful of languages, but I have contributed in environments that I don't "know."

To learn languages quickly, I think the best way is by learning the 'building blocks' languages are composed from. How are function arguments passed (value vs reference), how is memory management handled (garbage collected, reference counted, manual?). Learning these lower level language concepts then allows a person to approach a new language by saying "How does this new language X handle concept Y?"