Chouby on "[Plugin: Polylang] include/exclude sync of AFC Repeater Fields"

I am sorry but i don't know how custom fields are stored in ACF. Maybe you should ask to the plugin author. What Polylang does is to copy customs fields as known by WordPress. The filter pll_copy_post_metas expects fields names (as they are stored in DB). If a custom field is an array, Polylang copies the whole array.