This also exists in slav languages - as a child secret speech. It is easier to understand than to say (both pronounce and construct the deformed word)

I'm French. My grand father used this form of slang with friends when he was young so they didn't get understood by others.

I've leaved in many parts of France and never heard anyone use it, probably long dead?

I am a native French speaker and I never heard javanais (grew up in Paris city).

However, I had a teacher who was always saying "Do I have to tell you that in Javanais?" (sarcastically) when we were not paying attention to what she was saying in class.

While I dont think it is specific to a region, it is probably specific to an age: verlan is quite used among late teens / young adults, while Javanais is more used by childs / young teens. I used to speak Javanais with my sisters as a way to speak without our parents understanding (hopefully!).

It is not really a "slang", but I thought it was similar and worth sharing

This is why La Javanaise has so many av sounds (though it's not javanais)