Before using Entity Translation, I used the
Language switcher (User interface text) to allow the users to switch between languages on the header.
On the 'Configure entity translation' page, under the heading 'Enable a different language switcher', it states:
When the Entity translation module is installed, a second language switcher block is made available on the site called Language switcher (Content). Enabling this block rather than the default language switcher block, Language switcher (User interface text), will ensure that the language of the content is updated when the language switcher is used.
However, when I place the 'Language switcher (Content)' block in the header, it does not even show up. Switching back to 'Language switcher (user interface)' shows it again, but then it won't ensure the language of the content is updated using the languages switcher.