A new Google Translate feature allows users to improve translation quality: rating translations. There are three options: helpful, not helpful and offensive, but Google doesn't let you highlight the text that's poorly translated.
Another way to improve Google Translate is to click a word from the translated text and choose one of the alternate translations. You can also enter a better translation or hold down the shift key and drag the words to reorder them.
While Google Translate supports 64 languages, you probably use it for a small number of language pairs. That's the reason why Google highlights the languages you've recently selected.
[Thanks, Charlie, Alex]