If a player quotes a GM's meassage that contains a "Private to" message, does that message become revealed to the player quoting?

I have no idea. I always delete that sort of thing out on general principles. I tend to shorten quotes to make clear which part I'm responding to.

In reply to Jhaelin (msg # 1):

You may test it.

Just make a post in any OOC thread specifying it's to try that, then quote it and ask other players if they can see it...

Private lines are not included in quotes, be they 'Private to' for a specific player or GM, or private to language groups.

It will make quoted posts look weird, but the "security" of the private line will be maintained.