Just issuing an instruction to Google to translate the page rather than highlighting the text you want translated probably starts to try and translate the hypertext markup language used to create the page (all the hidden bits that set the page layout, text size, colour, and image positions). This of course results in nonsense.
When you highlight, or cut and paste text, you strip away all the hidden superfluous nonsense, so Google "knows" what you want translated. Computers do exactly what you ask them to do, not what you expect them to be able to do!