From the error message the software can not find the default langugae set. You need to set the default language to the language that you will keep before you delete the default langauge. No you need to upload the language folder again to the langauge folder on your server so if you deleted the default en-US you need to upload the folder: language/en-US to your server again or you can also rename the current langauge you still have on the server to en-US until you create it again thanks
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