You may have noticed that it is possible to receive the response portion of an API call in languages other than English.
How does Zendesk determine the language in which an API response is sent?
This is set based on the Language setting in the profile of the user that is making the API call.
Upon user creation, an individual user's language setting will be set to the default language of the account under Admin > Settings > Account > Localization. If you are unsure how to change the language of an individual user, check out this article.
If you're wondering how a language in a user profile may have been set, please see How are users' language set in Support?.