You are running into a known issue:
https://github.com/gohugoio/hugo/issues/9557
With this structure:
content/blog/foo/foo-2/
├── cover.fr.jpg
├── cover.jpg
├── index.fr.md
└── index.md
You cannot use this markdown on both pages:
There’s a workaround here; requires an image render hook:
https://discourse.gohugo.io/t/multi-language-images-and-page-bundles/37325/8