It's not possible to select base translation when translating Content.

Steps to reproduce:
1. Login as Admin
2. Add a new language (for example: Polish)
3. Create a new Content Item: Folder named Test
4. Go to Translation tab and start adding a new Translation

Expected:
It's possible to select "English (United Kingdom)" as as base translation

Actual:
Base translation cannot be selected. Clicking it does nothing.