If JSON contains a syntax error, when you click on “Manage TemplateData” button to load TemplateData editor, you will see
MediaWiki:Templatedata-errormsg-jsonbadformat, but it looks strange; current content:

Bad JSON format. You can cancel this operation so you can correct it, delete the current <templatedata> tags and try again, or continue to replace the current TemplateData with a new one.

This lets thinking there is 3 possibilities, whereas there is only two buttons: either you manually fix JSON or you let the editor to reset templateData. So, I think it would be clearer to remove “delete the current <templatedata> tags and try again” text, like that:

Bad JSON format. You can cancel this operation so you can correct it, or continue to replace the current TemplateData with a new one.

Also, “Replace” button (MediaWiki:Templatedata-modal-json-error-replace) would gain in clarity, in my opinion, to be renamed “Reset” instead (if so, adapting previous message again).