Can you maybe copy the entire string you see in GUI to a new post so I can look for it and see where in the source code appears (and if it appears in any of the translation folders).
I have checked and it seems that in 4.1,0 string is translated to a couple of languages. Here is the exact location of translation in question for translation en_US:
The way I would go about this is:
Download mautic release again
Backup current translation dir in your mautic instance /plugins/GrapeJsBuilderBundle/Translations
Copy Translations of GrapeJS from the new mautic source you download it.
Clear mautic cache (clear mautic cache by actually deleting files in var/cache/prod/ folder using rm command - cache:clear from bin/console does not work all the time in my experiance)
Login
Select en_US language on profile page
Logout and login again
Translation should appear.
Note: If you use any other language setting then en_US you will have to ensure that translation for the setting actually exists.
And finally if you fill like it, you can just put the missing string marked in link above into your current translation folder.