I'm a merchant who wants to use 'Spanish' for my customers, what should I do?

Spanish is one of the languages for which Chargebee provides translation out-of-the-box (see the complete list here), hence you need to enter the text (in Spanish) in the web interface wherever applicable such as

• Plans,
• Addons
• Custom Fields

and enable Spanish in the Settings > Configure Chargebee > Languages screen.