The JVM has supported alternative languages for years. Some of the better known ones include Groovy and Scala. Both of these languages have influenced Java in one way or another over the years, but, like any language, they are not without their problems. Many feel that Groovy doesn't perform as well as Java (though the invokedynamic bytecode instruction is supposed to have addressed that), and many find Groovy's more dynamic nature less appealing. Scala, on the other hand, suffers (fairly or not, depending on who you ask) from the perception that it's too complex. Compilation time is also a common complaint. Also, many organizations are quite happily using both, so they are definitely worth considering to see if they will work in your environment and for your needs.
While those may be great languages, we are taking some time here to see what's next, and there are at least two languages that seem to stand out from the crowd--Ceylon and Kotlin. We can't give each of these languages...