Change language into Vietnamese

As you have Vietnamese translation support, I want to change the plugin’s language into Vietnamese. I know that the plugin changes its language accordingly to wordpress’s language, but wp is not available in Vietnamese yet while this plugin is. How can I change the plugin’s language into Vietnamese while keeping my wordpress’s language English?
P.S. Also, I tried modifying the language files (*.po & *.mo) in the /languages/ folder, but nothing seemed to change. I also tried deleting the whole language folder, nothing changed either.

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