Language switcher to customise homepage missing

I’m using Polylang on two sites with the same theme (OnePress with advanced Theme Customiser for homepage content). I installed it months ago on website 1 and it worked no problem. Today, I installed it on the website 2 and the option to change languages within the back-end of the Theme Customiser does not display, which means my homepage is in one language only.

I have followed the same instructions: there is an English and French page which are linked to each other and use the Homepage template of the theme. In ‘Reading’, I have set one of those pages to the homepage and I can see the content in the Theme Customiser. However, I cannot switch between languages within the Cusstomiser.

On website 1, I can see ‘Language:Eng’ or ‘Language:Fra’ (‘Eng’ and ‘Fra’ are in their own white box) in the Theme Customiser. When I toggle between them, I can set different content in each section of the homepage. This setting is missing from website 2.

After playing with many of the settings suggested in other threads, I rolled back the website to yesterday’s version and started over. All Polylang settings are now back to default.
– Permalinks settings are “The language is set from the directory name in pretty permalinks” and “Remove language in pretty permalinks” with the checkbox checked for “Hide URL language information for default language”.
– Using a static front page, which is correctly set.
– Polylang settings are all default.
– All other plug-ins currently disabled.

This is time I’m donating to a not-for-profit charity website, so all help gratefully accepted!

The page I need help with: [log in to see the link]