Multilingual support does not work with country-specific language codes (pt_BR)

[ericof]

Describe the bug Plone, historically, used the country-specific language codes in the format pt-br (plone.app.multilingual, GenericSetup), but the translations in plone.app.locales and in Volto use the format pt_BR.

In Volto, adding the following configuration to config.js will break API calls to Plone (even if plone.app.multilingual is correctly configured to both languages)

    isMultilingual: true,
    supportedLanguages: ['pt_BR', 'en'],
    defaultLanguage: 'pt_BR',

Trying to rename the Volto locale to pt-br will break the LanguageSelector component and intl-locales-supported,

To Reproduce Steps to reproduce the behavior:

1. On the Plone server configure p.a.multilingual to use pt-br and en as languages (pt-br as default)
2. Create a new Volto package
3. Configure
4. Modify config.js and include the settings isMultilingual: true, supportedLanguages: ['pt_BR', 'en'],, defaultLanguage: 'pt_BR',
5. Start Volto
6. Access the Volto site

Expected behavior Calls to the Plone server would be successful

Software (please complete the following information):

• OS: Linux
• Browser Chrome, Firefox
• Volto Version 13.0.0-alpha.1
• Plone Version 5.2.4
• Plone REST API Version: 7.0.0

[ericof]

Implemented in #2415