There was an update to the To Google Translate extension a few days ago that temporarily changed how the user interface works. This was because the in-page overlay stopped working for a lot of users. See: https://github.com/itsecurityco/to-google-translate/issues/121. In the current version, you still can select text, right-click, and call up the extension from the context menu, and your translation should open in a new tab.