The good news, and perhaps also the bad news, but in your chosen context the good news, is that Javascript, Python, Ruby, and to a lesser extent, Objective-C, are all essentially the same language. You're probably not as split as you think, and if you are going to stick within that paradigm you at least have perspective relative to someone who only knows one of them. You are right that you should really learn one of them deeply, but it hardly matters which, and a great deal of the deep knowledge will transfer fairly well. (Again, Objective-C being the odd man out a bit, but still in the same broad family.)
It seems to me that there's a developing consensus even in the deployment ecosystems. Everybody's growing towards what I might call a file-system-virtual environment per application, so you don't get conflicts in the "core" OS, automatic dependency management, etc etc, and it seems like the libraries grow ever closer to each other. What is one year some bizarrely-awesome library in one particular language is two years later copied in all the others. (Which is a good thing.) That whole family of languages is all headed for the same basic local optima, which is the bulk of why I would claim they're all essentially the same language.
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The good news, and perhaps also the bad news, but in your chosen context the good news, is that Javascript, Python, Ruby, and to a lesser extent, Objective-C, are all essentially the same language. You're probably not as split as you think, and if you are going to stick within that paradigm you at least have perspective relative to someone who only knows one of them. You are right that you should really learn one of them deeply, but it hardly matters which, and a great deal of the deep knowledge will transfer fairly well. (Again, Objective-C being the odd man out a bit, but still in the same broad family.)
It's more the ecosystems than the syntax and semantics.
Bundler, gem, pip, npm, blah blah blah. It's a lot of stuff to grok if you want to get anything done in the 'right' way.
It seems to me that there's a developing consensus even in the deployment ecosystems. Everybody's growing towards what I might call a file-system-virtual environment per application, so you don't get conflicts in the "core" OS, automatic dependency management, etc etc, and it seems like the libraries grow ever closer to each other. What is one year some bizarrely-awesome library in one particular language is two years later copied in all the others. (Which is a good thing.) That whole family of languages is all headed for the same basic local optima, which is the bulk of why I would claim they're all essentially the same language.