The default locale in the Text File Properties dialog is based on your OS Regional settings locale. For example on Windows, Go to the Control Panel > Region settings > Format tab. Then set the format to your preferred English locale. Then when you restart Tableau, it will default to English in the Text File Properties and the workbook locale will also default to English settings. Unfortunately, this will also update the locale for your OS and all other apps on your machine. There is currently no way to always set the default Locale to another region without updating the settings on your OS.