Paratext uses the SLDR (a superset of the CLDR) as an online resource for getting information about languages that Paratext doesn’t know about to keep users from having to enter in all the information about their language. I was assuming that the user’s project had lost it’s language definition file and so Paratext was attempting to get the language information from the SLDR.
However, looking closer at the error message, it looks like the English language file somehow got deleted and Paratext is attempting to go to the SLDR to get it, but it is failing to download the whole thing (it’s big) and is then crashing when trying to read the incomplete data.
The user can probably uninstall and then reinstall Paratext 8.0 to fix the issue.