Hi
1. If I go to SEO > Titles & Metas, I can insert values for the meta title and meta description tags for every post type, whether native (post, page) or custom, which I have done.
2. If I go to Settings > Languages > Settings, I can indicate which custom post types I want to be able to translate, by checking the corresponding boxes, which I have done.
3. If I go to Settings > Languages > String Translations, I can provide translations for all sorts of strings, including those automatically generated by the WordPress SEO plugin. This is where I expected to find the strings for the meta values of my custom post types, which isn't the case.
Now I am totally aware that this might also have something to do with the WordPress SEO plugin not generating translatable strings for custom post type, so it might be a non-issue with Polylang, but I'd rather give a shout here than with Yoast, since their support isn't easy to deal with.
Thanks!