For Joomla 3 at least, I had to add code to attempt to load the English language strings first, then the desired language file. This wasn't included behavior in J3, and I'm unsure of whether it's available in J4 now. Can you try renaming the file from es-ES.com_jfbconnect.ini to com_jfbconnect.ini to see if the default English string gets used?
When a language string is not yet translated Joomla 4 should load the default English Language string, Can you confirm ?
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