When I write an email using the Android mail apps, how can I send it 2 hours later? When I write an email at 2:00 AM or 2:59 AM I don't want to send it at that time (because I want to go to sleep), but instead tell Android/GMail to send this email a few hours later, because of timezone differences.

Right now I can only send immediately, not planned email. Are there any GMail apps which can do that, or other apps which can use this concept and send the email via GMail in Android?
check this article out.
Using m.boomeranggmail.com you can sign in with your gmail account and schedule delivery times for your emails. 
As of now there is not a standalone app, only the mobile web app. But it will do what you're wanting!