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The request is to have a web form fully available in a language other than what the account is set up as for all recipients/signers.
The language for non-configurable "system" labels and descriptions (these are error messages and other non-configurable text (for example, "Loading…", Finish Later, Next, Back, etc.) on the form as well text in the header/footer of invitation and completion emails) is determined when the web form is initially created based on the form creator's language settings in their Docusign account.
Updating the web form configuration file as well as branding worked for the first recipient/signer to have their web form fully available in another language other than the account. However, this did not affect the experience of the second recipient/signer - their non-configurable "system" labels and descriptions were still only available in the language of the Docusign account.