What to do when a journal publishes a paper plagiarizing one's work but doesn't care about it?
Document everything.
If you can't get a reply from the editors of the journal, write to the publisher. Or if it's the journal of a learned society, contact them.
You probably can't act alone, but you may get more powerful allies (deeper pockets) on your side. There are others around you who have a vested interest in your work.

In particular, the journal where you published your work… given that you transferred them the copyright, I suppose they might act in their own name against the plagiarist. Even if it does not escalate all the way to a legal action, the publisher of the plagiarized work might respond (or respond faster) to a well-known publisher.

Your employer may also have an interest in helping you enforcing your copyright. They may also have a legal department who can advise you on this matter.