Option Group element is not translatable

Problem/Motivation

Option Group element is not translatable

'Group 1':
  '1': 'Item 1'
  '2': 'Item 2'
  '3': 'Item 3'
'Group 2':
  '4': 'Item 4'
  '5': 'Item 5'

Steps to reproduce

1. Open a webform with select element
2. Group options
3. Translate Group labels
4. Save translation
5. Edit translation: Group label translations are gone

Proposed resolution

Save group label translations

Remaining tasks

User interface changes

API changes

Data model changes