Cannot overwrite default translations in documents anymore (in french wiki)

To reproduce:

• set a wiki to unilanguage, default language "fr"
• create a document with translations, say Sandbox.Translations – the document will have the following language settings

  <language/>
  <defaultLanguage>fr</defaultLanguage>
  <translation>0</translation>
  

• set this document as localization document in XWikiPreferences in the localization section
• putting new translation keys in this document and using them all over the wiki with $services.localization.render(key) will work fine
• we want to overwrite the default login and logout texts, defined in ApplicationRessources_fr.properties
• we put

  login=New login text
  logout=New logout text
  

  in Sandbox.Translations

• nothing changes on the UI for the login/logout links

The cause seems to be the locale of the translations document Sandbox.Translations which is "" . The way translations work is that translations in the exact demanded locale (fr) have priority over default translations, and if a translation for the exact locale is found, the default one is not taken into account. In this case, ApplicationRessources_fr.properties has exact fr as locale, and Sandbox.Translations has "" as locale, even if, indeed, Sandbox.Translations has higher priority than ApplicationResources (as XWIKI-8881 says).