Nicest thing to say in Japanese - Otsukare (お疲れ)

Otsukare means "fatigue". In Japan, it's highly respected to work yourself to exhaustion. 

When people leave the office at night they say — otsukaresama deshita (you're tired). It's the nicest thing you can say in Japanese. 

Source: http://www.japan-talk.com/jt/new/7-most-important-Japanese-words/