en-US.
Auto-Detection
Passmulti to enable automatic language detection. The aliases auto and auto_detect are silently mapped to multi.
Engine Support
Supported Languages
For most engines, Telnyx passes the language code directly without validation. The supported set depends on which engine you use. Speechmatics is the exception — Telnyx accepts shorthand codes for bilingual/multilingual packs and maps them to the provider’slanguage + domain configuration internally.