Maybe this will illuminate you.
This site studies the top programming languages by usage.
C is #1. (As it should be.)
Guess who's #2?
Well moved from C++ to Java (In college) and found the transition easier, then I had to temporarily move to C++ from a project and initially it was hell.
So if you already have worked on C++ and have put in some amount of effort in Java, I would suggest sticking to Core Java.
ALso I do not understand why you feel Python or PHP would be any easier to learn than Java, if you try to cover the same concepts in those languages, I think it should take a similar amount of effort.
Another thing I feel is you might be looking at this the wrong way. You should be focussing on your Data Structures and design skills (Object Oriented Analysis and Design), you might learn and become familiar with Collections API, etc but in the end we programmers are just problem solvers, good interviewers usually try to see your approach on analysing a problem and designing solutions for it.
Finding out whether to use a HashMap or a TreeMap or TreeSet or HashSet for a given situation in your data structures is easy (either google or just read the javadocs).
It is the how you think about the problem which is imp not just for interview, it will help you even on the job.
Also, multithreading isn't just a Java thing. If a language and its libraries don't implement support for multi-threading then they're no use in the real world of software deveopment.
Multi-threading is conceptually difficult, and can be very hard to wrap your head around. Java's support for multi-threading is as easy as it gets, mainly because it has excellent library support for amny of the more common ways you would use mutiple threads in practice (thread pools, swing worker etc etc). You won't find a popular language that does it properly and makes it any easier than Java.
As an aside, it should be mentioned that cross-posting to multiple forums is generally frowned upon, as it leads to repetition of effort, and also clutters up the fora. If you feel the need to cross-post at all - and you almost certainly shouldn't unless you are certain that it will make a difference - you should at least give a link to the other fora you've posted to.