If you really want to challenge yourself, how bout an Assembly language? Or machine code! (It’s possible!)
But in all seriousness, how about Kotlin? I believe Kotlin is meant to replace Java for Android development, so it might be useful to pick up. Or Swift, for iOS.
The last language I learned was Crystal because some of the syntax was very similar to Python, but I’ll give Kotlin a shot and see if it’s a good fit for me.
Edit:
I tried Kotlin, but when I tried to run the code, it just sat there for a while and did nothing?
Hmm… just tested it, it took a couple of minutes but it did, run. But it took way too long… Might be worth a bug report. Either way, try checking out Swift.