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This is a strange thing.

But According to Django:

zh_TW is a Locale Name: The language part is always in lower case and the country part in upper case. The separator is an underscore.

zh-tw is a Language Codes: It is generally represented in lower case, but the HTTP Accept-Language header is case-insensitive. The separator is a dash.

It is better to user those codes with all lower case (eg. zh-tw)

more info is available at Official Django Documentation.