I was expecting responses like yours. Thank you for wasting your time on your very useless post.
Hold your horses there.... I think Alvaro makes some good points, and altough they might not apply to you, nothing in your first post really pointed in the direction that you had any expierience in running real MMO Game Servers (well, I am still not convinced you are talking about real MMO Cluster Servers and not just game servers for normal multiplayer games, which can be worlds apart because of scaling).
Let alone that you have the time, expierience, large team, money or other resources needed to really be successfull with your MMO endgoal.
But I understand that this was not your question... just try to understand that you are number 193492398 in theline of people coming here asking questions about MMO development (and your question, in the end, was about that: "What language to choose (with the endgoal to use it for MMO development)"), with little to no expierience to show that they are capable on really reaching their lofty goal (again, you might be different, but your TO Post does not read like that). While I don't think Alvaros post was that helpful or answering your questions, he makes some valid points you should consider (and, if you think they don't apply to you, safely ignore).
To come back to your question: I am pretty sure for Game dev C++ will be the more popular choice than Java. I know there are enough people cheering for Java, there are Java engines out there and then there is Android development, but for raw performance C++ will certainly still beat Java.
If that is important to you, only you can answer.
As soon as your game is not using cutting edge graphics, is not very CPU intensive and performance hungry, the question about language becomes a moot one. Use whatever you feel comfortable with. If you have C++ expierience, continue with that. Java will give you not enough benefits over C++ to justify re-learning everything.... of course, learning a new language is an easy task as soon as you are an expierienced developer.
The question should be rather, what Middleware / Frameworks / Engines to use to prevent re-inventing the wheel. Of course, if you like to develop your own engine, have fun and do just that.
But its a lot of time and work for little to no gain, that will take you off track from your real goal: developing your game.
So I would suggest you really take a step back and ask yourself: "Do I want to develop an engine or a game?" .... for learning purposes, to become a good engineer and get some portfolio pieces and streetcred for an engineer position in a game studio, the first path certainly is a good one.
If your own game is your only real goal, get an existing engine, research a good Network middleware, and let other people do the low level plumbing for you. Especially with your lofty aspirations, developing all the necessary netcode and DB Stuff will take you years for no real gain over already existing solutions.
@Alvaro: gave you a +1 to even out the score as I don't think you deserve the downvote.