In mediawiki 1.31.1-release downloaded here : https://releases.wikimedia.org/mediawiki/1.31/mediawiki-1.31.1.tar.gz
If the parameter $wgShellLocale in LocalSettings.php is set on fr_FR.utf8 the resizing of images not work and return only the link of images
The reason is located in img tag. The "srcset" for the medium size of images is 1.5x of pixel density but with fr_FR.utf8 it is 1,5x because there is translation in french.
So the browser don't understand this information and return the link of images without display the images

Steps to reproduce:

1. Install and default configuration of mediawiki 1.31.1 provided by this link : https://releases.wikimedia.org/mediawiki/1.31/mediawiki-1.31.1.tar.gz
2. In LocalSettings.php set $wgShellLocale="fr_FR.utf8"
3. Connection with an account

4.Create new wiki page

3. Use the insert image method and upload an image on the mediawiki
4. Try to display image by adding the mediawiki code : [[File:xxxx.png|50px]]
5. Save the change and go to read mode on this page

Actual Results:
Only the link of image is displayed but not the image. If I do right-click and inspect element, I see in img tag the "srcset" with 1,5x instead of 1.5x

Expected result:
The images display with resizing