Cannot create a translation for a child locale as long as the translation for the parent locale exists

Follow these steps to reproduce:

• Enable multilingual and configure 3 supported languages: English, French and French (Algeria)
• Create a page with default language English
• Translate the page in French
• Try to create the French (Algeria) translation
  • From Wiki editor the UI shows the expected information, but after you save you realize that you actually updated the French translation, not the French (Algeria) one
  • At the same time the link to create the French (Algeria) translation from the Information tab, which should take you to the inplace editor, doesn't work: the inplace editor doesn't load

This problem doesn't reproduce if the parent locale (e.g. French) is the default locale of the page. I suspect the problem is related to the fallback we do when a translation is missing: if the child locale, e.g. French (Algeria), is missing we fallback on the parent locale, e.g. French.