Okay, we have done some testing and the site-wide header image as selected in the customizer is displayed correctly across all languages, so you shouldn’t need to upload a duplicate image for each language.
The header image can also be changed per post/page in the post/page editor with the ‘Featured Image’.
Let’s say you have ‘Language 1’ and you create a page ‘Page 1’ for that language. If you upload/select a featured image, this will be displayed as the header image for ‘Page 1’ in ‘Language 1’.
Then you create a ‘Language 2’ version of ‘Page 1’, if you leave the ‘Featured Image’ empty, the site-wide header image will be displayed for ‘Page 1’ in ‘Language 2’. So, to have the same ‘Featured Image’ you just need to select the same image, no need to reupload a duplicate image.
Hope this helps and that I have explained it adequately.