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Why 2 [lang].csv in frappe and erpnext

I found there are 2 lang files in frappe and erpnext, eg. zh_tw.csv zh-TW.csv.
Their format are different, seems zh_tw.csv format correct, but frappe always load zh-TW.csv file.
I select language 繁體中文, still load zh-TW.csv and it’s wrong translation.

How to fix this problem?
Thanks