Theme name translations are not shown correctly

How to reproduce:

  1. Install a brand new Discourse instance. Choose French as the default language, and Neutral as the theme => everywhere in the interface, the theme name appears correctly as “Neutre” (French translation of Neutral).

  2. Run the wizard again and choose the Dark theme => everywhere in the interface, the theme name appears incorrectly as “Dark” (instead of French “Sombre”, as defined here).

  3. Go to the settings and switch the default language to US English => everywhere in the interface, the first theme name is still displayed as “Neutre” (instead of “Neutral”).