How to reproduce:
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).
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).
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”).