Since versions Responsive 4.4, Mobilia 3.4, Retina 2.1, Parallax 1.1 we introduced support for theme translations.
This allows you to easily update any text that appears throughout your shop.
Start by going into Online Store -> Themes -> Actions -> Edit languages
By default, we have fully translated versions of our themes in English, French, Spanish, German and Portuguese (Brazil and Portugal).
Click here to see Shopify's official doc on translating your theme.
Note that also in this area, you will be able to change the various text that can be found in your shop such as "Sold Out", "Add to Cart", and "Checkout".
If you need to support multiple languages in your shop at the same time, you will need a Shopify App to provide this functionality to your customers. This will allow your customers to select a language when viewing your shop.
You could also consider using the Google Translate widget in your shop to have Google automatically translate your shop. You will find Shopify's instructions here.