I don't do it on open source projects, but I have always found the best way to learn a new language is to find a project and add to it. It also makes you an awesome employee because you are happy and able to learn on the job
The only thing time I would say it is different is if the inheritance pattern or some other major feature is very different from what you know OR it is your first programming language. Then it's best to do some research on differences or learn the basics before you dive in! I have learned multiple languages this way.
I think I am going to start with open source next time I want a new language though, it sounds fun!
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