Polylang
Depending on some of Polylang’s settings picu collection might not work as expected.
1. Default Language
Once you have configured a default language in Polylang, collections will always be displayed in that default language, even if another language is defined in your General Settings.

2. Media Translations
Activating Media translations might break picu collections.
After activating Media translations for the first time, images will not be show when viewing collections. You have to first assign a default language to existing media. After doing that, your collections will work again.
Beware: If you then change the default language (after assigning the default to existing images), your collections will stop working!
We are currently working on a fix. Please get in touch when you want to use Polylang and picu on your site.
WPML
1. WPML Media Translations
If activated, the module WPML Media Translation will create entries for all media uploaded to your site to make them translatable, including all of the proofing photos uploaded to picu. This can lead to images showing up in places where they shouldn’t and can prevent removal of those images once a collection is deleted.