I don’t have a lot of experience with WPML, but there shouldn’t be any conflict.
As far as I know, WPML stores each translation as a different post, with a different ID, so you need to check what is the actual parent of your images. You can check this quickly in the Media Library, in list view. The images will be removed only when the parent post is deleted. Depending on how you upload your images, the translation you are removing might not be the parent post.
Take this scenario:
Post X is translated into language A (ID 100) and language B (ID 101).
When you upload your images while writing in language A, the parent of your attachments becomes ID 100. If you remove language B, your attachments won’t be removed because the plugin looks for child attachments for the post with the ID 101. They will only be removed when you delete the translation in language A, when the returned ID is 100.
Unfortunately, in WordPress, a media file can have only one parent and there’s no way of checking if an image is used in multiple posts without huge performance hits. It would mean doing multiple serialized scans of your database for EACH media file in your media library. And that would cause timeouts on most websites, especially for those on restrictive hosts.