I'm a Chinese programmer and I love Greenshot very much, but the biggest problem is that I can't input Chinese in a text container. So I checked out the code from SVN and made a modification to solve the problem.
Actually it's pretty simple, just append "textBox.ImeMode = ImeMode.On" after you create a TextBox. As doing this, users can input Chinese and Japanese as well in the TextContainer.
I've attached a modified file so that you can put it into your code base conveniently.