Locales for facebook are not correct

[kevinmu17]

Description

when debugging with: https://developers.facebook.com/tools/debug/og/object/

I am getting this error:
Object at URL 'https://sitename.nl/' of type 'website' is invalid because the given value 'nl-nl' for property 'og:locale:locale' could not be parsed as type 'enum'.

I think the problem lies in the "-" and should be transformed to "_" (dash to underscore)
https://developers.facebook.com/docs/messenger-platform/messenger-profile/supported-locales/

Steps to reproduce

1. install SEO plugin, add {% hook 'seo' %} to your twig template
2. insert your site url in https://developers.facebook.com/tools/debug/og/object/

Additional info

• Craft version: 3.0.25
• SEO version: 3.4.4
• PHP version: 7.1.21
• Database driver & version: MySQL 10.1.35
• Other Plugins: N/A