If you are talking about panasonic cameras specifically, who are the people buying panasonic cameras from japan? Any examples to share? If you are talking about non-japanese, my guess is they either 1) can read japanese 2) bought by mistake... haha.
If you are talking about cameras in general, I think only Sony and Panasonic are locked into Japanese language. Sometimes they may have export versions that may have English menu option, but those will cost more over the domestic Japanese-only model.
People sometimes willing forgo warranty for price savings. You have to figure out whether it's worth it to you.
Basically, if you walk into a shop to buy a gx8 in japan, you will likely be faced with two options:
1) A domestic model (with only Japanese language option available)
2) International/overseas model, assuming they have it in stock. Shops catered to tourists should have it. (with the option to change language to english and other languages - similar to what you can buy in SG)
The international/overseas model will be more expensive than the domestic one. So at that point, I'm not sure if you will save any moneey over buying local. If the savings are significant enough for you to forgot local warranty, go ahead.
Read this thread:
http://www.dpreview.com/forums/thread/3958440
Panasonic also states on one of their sites: