Why is German language appearing on my iPod unexpectedly?

I don't believe that's "a copy and paste setting", but simply either the word that's in the clipboard (from where the paste comes) or it's a predicted word that the application thinks you want to type.

If it's a word in the clipboard, then that's what you've copied, even if you have not realised it.

Your keyboard, as shown in your screenshot, is not in German and I'm not aware of a setting for copy and paste. You copy something and it pastes what you copied. That's it.

Presumably, if you tap the word Einsetzen (as shown in your screenshot) that's what appears in the message field (because you've pasted it in), but if you instead type in a word of your choice, that typed in word is what appears in the message box. You could try an experiment by copying a specific word (that you will know you've copied and that you remember when you open this application) from somewhere and then see what appears in that box when you do that.