I haven't actually seen java in package names, but I can easily imagine the conversation where some over-planning architecture astronaut decides that the project may need code in another language in the future (without bothering to worry about if that language even uses Java style packages) and so they better make a directory that is specific for the Java code so they can keep the other code separate, if they need to add more languages in the future.
I've not seen it directly in the package names, but I've definitely seen project hierarchies with source/java even though no other languages were used (or going to be used, for that matter).
To be fair, src/main/java is often the default places build systems look for code. I'd rather use that then complicate my build scripts.
There's also src/main/resources, src/main/webapps, and possibly other dirs that can go in there that aren't other languages.
What if you have projects in other jvm languages?
never having been in such a situation, I wouldn't know, but this is the first time i'm hearing of people putting java within the package name (for something that's not part of the java namespace)