Got this solved, thanks to Polylang's support:
- Create a 'polylang' folder in wp-content (or use existing 'polylang' folder)
- Upload the wpml-config.xml file (see below link) into the 'polylang' folder
- In your Wordpress admin, go to to Languages > String translations and search for 'frontpage_select_page'
- Replace the existing number in the ID field of the foreign language with the ID number '112' (see screenshot received from Polylang in below link )
- Save the changes
And this should do it! It worked for me, with a site in English and Japanese. Note that we're using Project Thumbnails for our home page (not Thumbnail Grid or Element Grid), and had to manually add the Japanese thumbnails to the Japanese home page.
XML file and Polylang's screenshot are here: https://tinyurl.com/y42xtz8b
Hope this helps!