There is indeed a disabled button and I figured that might be trying to communicate that it was disabled, as I saw in your response to another user about two weeks ago.
I will experiment with the other modes, though the path that seemed most intuitive was to use Desktop mode and to just identify the part of the screen (in a non-fullscreen application, at least in this case) that to be translated. I am unsure whether a typical user would be confused if the GameTranslate window were to also be captured initially, since they would likely just readjust their settings to keep it away from the text to be translated.