Your shop language is a separate option from the language in which you view the site.
Your language preference for viewing the site is under You > Account settings > Preferences. This is a personal preference, and what you choose here won't be reflected in your shop or to your buyers.
You can also change your viewing preference by clicking the language at the bottom of the site.
When you register as a seller, the first language you register is your default language.
To enroll in another language, or check your default language, click Manage languages on this page: Shop Manager > Settings > Languages and translations.
You'll also be able to manually translate your shop and listings to other languages aside from the one you set as your default.
If you only use your default language, buyers who speak other languages will still be able to see your listings in their language. We automatically translate listings so that shoppers can read listings which weren't listed in their language.