I think it's definitely Java. Java has been one of the leading languages for 20 years, so Java is persistent and in great demand on the job market.
However, knowing just one language is no longer enough today. So that you can feel safe on the job market, you should at least know SQL. Knowing databases in general is very helpful when looking for a job.
In general, it is very difficult to say which language will prevail in the future. Many languages are fast moving, like the computer life itself ..
But Java is a good choice ..