The Polylang plugin falls under third party plugin category as the theme does not recommend this plugin .
However when we tested the plugin with the theme the plugin works fine .
As for the issue you have mentioned , actually it is not the issue . Except for the default language you have to upload your required images again as default image will only work for default language .You need to create post type and upload image for each language as all of these will be language specific . It is not the issue for the plugin feature .
When you upload the image in header image it displays fine in pages/ post for any language .
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