Pontoon mixing dash and underscore in locale folder names

[flodolo]

Added a new locale to VPN Client (nn-NO). To my surprise, Pontoon created a nn-NO folder instead of nn_NO, which was the style already in use in the repository
https://github.com/mozilla-l10n/mozilla-vpn-client-l10n

I don't know the practical impact of this, but we should maintain consistency.

I would also like to understand how to fix it without impacting existing data in Pontoon.

[flodolo]

I have a feeling this might date back to #3312