Copy the file called wpml-config.xml from you parent theme’s folder (e.g. zerif-lite folder) into the child theme’s folder, open it and change the line:
<key name='theme_mods_zerif-lite'>
to:
<key name='theme_mods_zblackbeard'> (or your child theme’s folder).
After this step, you should be able to translate the strings using Polylang->Strings Translation
Kind Regards.
