Describe the bug

When the portal's default language is English, we are unable to add a new language. The following error occurs:

The selected default language must be in the list of the field 'Available languages'

To Reproduce
Steps to reproduce the behavior:

1. Create a portal with English as the default language.
2. Go to Site Setup -> Language.
3. In Available languages, add a language.
4. Save

Expected behavior

I hope it will be possible to save the form in the new language.

Software (please complete the following information):

• OS: Ubuntu
• Browser: Chrome
• Volto Version: 19
• Plone Version: 6.1.3
• Plone REST API: 9.x

Additional context
This can be reproduced at https://volto.demo.plone.org/