Hi Xavier,
sorry that I was wrong with the file comparison, don't know if I got some "cache involved misunderstood", I redownloaded it right now and is diferent form my yesterday evening download.
Anyhow:
I'm not able to see this checkbox in the CSV products filter and also not in the Action Export CSV file (I understand it has to be in the filter).
I checked both in the existing Mass actions and creating new ones from scratch.
My backend interface is in english. I could move to french if needed.
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UPDATE
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I made a new clean install of HikaShop with most recent version (fresh download) and put the massaction-php in classes folder. Uploaded the plugin form the forum.
I can still not see the 'allow the creation of the products not already in the shop' checkbox.
So I assume this lack is not generated by any peculiar thing in my test sites. Hope this helps guiding your team to find out.