After searching through all support topics, I found the issue causing this. It’s the Jet pack lazy loading feature. I deactivate it at Jetpack → Settings → Performance and now the images are changed according to the language. One big request to the developers of this plugin: Please fix this bug. Lazy load speeds up the website, especially when there are many pictures. I would appreciate it very much if you could notify me when the issue is fixed so I can reactivate the feature. Thank you in advanced!
