Today I translated the Casper theme into Arabic language and made it RTL and it accepts multiple languages across locales easily.
Upload theme from Dashboard, Use Ghost settings page to Set the language/locale which is used on your site. (theme file in
Translate to our language
Ghost’s front-end and themes are fully translatable by enabling a publication language in the setting in Ghost admin, and using the translate helper to wrap around any plain text in your theme. Inside the locales folder, add target language files for each translatable language used on your site. For example
locales/ar.json for Arabic and
locales/fa.json for Persian (Farsi) … etc.
You can contribute to translating the template into your language, make a fork for the template and add your own language according to A valid language code must be used.