I have investigated extensions like uBlock and Greasemonkey and they do not have the welcomeMessage key so I guess it is not mandatory, why does this appear in my case?
Mmm, I don’t see that value in your extension. My best guess is that you accidentally pasted “__MSG_welcomeMessage__” into the Summary textbox when submitting it on addons.mozilla.org.
Try visiting https://addons.mozilla.org/developers/addon/cliproard/edit, then using the locale selector in the top right to switch to “Español · es”.
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You’re absolutely right, that was the problem 
I don’t know at what point that happened, thank you very much.
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Happy to help 
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