I usually run the French version of Rhino 5, but I also installed the English one.
When I launch multiple 3dm files in once for the Windows explorer some are opened in French and others in English.
There might be something to investigate on …
Hi Baptiste - how are you opening the files, exactly? Does it make any difference how you open them? (e.g. double-click from Explorer, Open from a browser after a download, drag and drop onto the icon, etc.)
It happens when I select several files in Explorer then press Enter.
If I wait for every sessions to be fully open before launching a new one, it works fine.