your data can disappear at any time
Don't expect to use LINE like you would any other messaging app. Your data is subject to being wiped out at any time. Even though all your data might exist in your device backup, when you restore it, you're likely to have the app tell you that you've signed in on a different device and that it is deleting all your data for no reason. What's more, it will probably also overwrite your iCloud backup afterward with only the most recent two weeks' messages that it restored from their servers. It's literally insane how poorly implemented their handling of your data is. Do your Japanese friends a favor and convince them to switch to something else, whether it's iMessage or WhatsApp or whatever.