We have integrated the DocuSign multiple languages (English, Chinese Japanese) in our system, and it works fine. But when a user used English to sign a contract and send it to the admin, the admin also received it in English. After both of them finished signing, the completed email notification is shown in different languages. For example, the admin will receive the subject, content, and footer all in English. The user will receive the content in English, but the subject and footer in (Chinese).
Language in DocuSign is, for the most part, dictated by the end-end-user. Meaning, the signer can choose what language they use when they sign a contract. So, if you build an integration and you do it in one language, and then your customer who is sending using your integration is using another language, neither of these have anything to do with the language that would be shown to the signer if they choose to use DocuSign in yet a third language. That is, assuming all involved have an account, where they can log into DocuSign and change their language preference. As a sender, you can change the language preference for the emails that are sent, but you cannot decide which language would be used to actually sign the document.