I'd go first to node to understand the basis about rest, http, db, etc. And then I'd swap to python/java. Even scala if you are brave enough.
I have to say though, for a frontend dev the big point is not to be able to build apps using the newest libraries. The challenge is to contribute to this libraries or even build your own one from the ground, point that it will require to have some understanding of CS.
I think that it's not for those people that know how to modify the css code of the bootstrap theme. They also offer a nice UI and a useful tips of design to create your own theme.
I have to say though, for a frontend dev the big point is not to be able to build apps using the newest libraries. The challenge is to contribute to this libraries or even build your own one from the ground, point that it will require to have some understanding of CS.