I think I've figured this out now but in case anyone else needs this. It's controlled through the blocks.

I made 2 mega sliders. One for English and one for French. Then in my Drupal block admin page I just set the English Mega Slider block to display when the site is toggled to English and the French block for the French language.

Structure > Blocks > Configure > Languages Tab :  'Show this block for these languages'