If you look at the user-role-editor/lang folder (free version), you will not see any translation file there except a .pot one. Plugins from wordpress.org repository use the centralized resource to get the translation data.

URE Pro currently uses its own local translation files. So, in order to translate URE Pro to the French you need to place translation files to the user-role-editor-pro/lang folder. Exactly as it’s done for the German translation.