"Best Rapid Development Language To Learn Today?"
Well sure, you could try using...
"Ideally, I'd like to learn a language that has web relevance, mobile relevance, GUI desktop applications relevance, and also that can be integrated into command-line workflows for data processing—a language that is interpreted rather than compiled, or at least that enables rapid, quick-and-dirty development"
Ah, um, hmmmm.
Look, I'm going to give you a protip about us "young folks": You seem to be under the assumption we are masochists. We are not.
If there was such a language, we'd all be using it daily for our development. The reason we use the tools we do are because they are the easiest tools for the job we do. Do you want to learn the most relevant tool for mobile development that is also the easiest? It's likely the one everyone else is using. If there was some hidden shortcut to do highly relevant development very quickly for every single platform, we'd all be using it. I don't like writing more code than I have to for the fun of it.
It sounds like you're trying to double dip here. You want a language that you can use for data modeling, but on the side you want it to be usable for about every single other arena for software development. Again, us young folks are already taking the easiest path. If you want to hit all those targets as well, learn the same languages that everyone else has already determined are the easiest paths, or keep focused on data modeling. The needs you're trying to specify for each language are totally different. Data modeling likes interpreted, but mobile strongly avoids the overhead of interpreted languages. All your requirements are exclusive, which is why there are a bunch of different languages and APIs to begin with.
Matlab is one tool I see frequently used by engineers who are mostly data modeling focused. For each of the other focus areas you've mentioned, I could probably list off several languages, usually with no overlap to the other platforms. Java possibly comes close, but you're not going to cover all your platforms, and it's not interpreted.
And to be honest, what you really haven't even scratched the surface of is that even if there was a language that covered all those platforms, you'd need to actually know all those platforms. Know the ins and outs of code signing on iOS or the Mac? Permissions on Android? 32 bit vs. 64 bit differences on Windows? The specifics of a bunch of different web browsers? If not, a language that covers all the platforms won't get you far anyway.