This is a follow-up question from my previous post Salesforce Apex Toolkit cannot override Email subject from Docusign Envelope template
Basically I needed to disable option “Custom email and language for each recipient” so that I can dynamically set the email subject and text using Salesforce Apex Toolkit.

Now the issue is, the email is always sent in English.

How do I set the default email language, with / without Salesforce Apex Toolkit?