A Postcrosser in New York sent me a card with these United Nations stamps.
The flag series was from the 1980s. The air mail is possibly from the 1960s, judging by the price, and the Human Rights one is from 1989. Why it is written in German, I do not know. In English, it is about the 'right to effective judiciary' - Everyone has the right to an effective remedy by the competent national tribunals for acts violating the fundamental rights granted him by the constitution or by law.
United Nations stamps have been issued since 1951 and have different themes than the regular stamps. You need to go to the UN building in New York, or Geneva or Vienna, to buy them and they must be mailed from their own post office. This could be partly why they are not considered very collectable.
see more united stamps at Sunday Stamps