Multilingual Auto-Detection

Automate with AI3 min readUpdated 2026-03-14

AI Voice Agent and JustCall AI now support multilingual auto-detection — the system identifies the language a caller speaks and responds in that language automatically. No manual language selection needed.

Available on: All AI Voice Agent plans.

How Auto-Detection Works

When a caller speaks, the system:

Detects the language within the first few seconds of speech
Switches the agent's response language to match
Maintains the detected language for the rest of the call

If the caller switches languages mid-call, the agent adapts.

Supported Languages

AI Voice Agent supports voices in 14 languages. The auto-detection system covers these languages and can identify the caller's language regardless of which voice you have selected.

When the caller speaks a supported language, the agent responds fluently in that language using the appropriate voice characteristics.

Setting Up Multilingual Support

Option 1 — Auto-Detection (Recommended)

Auto-detection is enabled by default. The agent detects the caller's language and responds in kind. No configuration needed.

Option 2 — Fixed Language

If you want the agent to always respond in a specific language regardless of what the caller speaks:

Go to Dashboard > AI Voice Agent and select your agent.
On the Behavior tab, add to the system prompt:

"Always respond in English, regardless of the language the caller speaks."

For businesses that serve specific language communities, add a language choice to the call flow:

"Thank you for calling. For English, stay on the line. Para español, diga 'español'."

Knowledge Base and Language

The knowledge base content language matters:

If your knowledge base is in English but the caller speaks Spanish, the agent translates knowledge base answers into Spanish on the fly
For best accuracy, provide knowledge base content in the languages your callers use most
Q&A pairs are particularly effective for multilingual support — create pairs in each target language

Best Practices

Test in your target languages. Call the agent speaking each language you expect to support and verify the responses are natural.
Voice selection matters. Some voices sound more natural in specific languages. Test and select voices that match your primary languages.
Keep prompts language-agnostic. Write behavior prompts in English — the agent applies them regardless of the conversation language.
Consider cultural context. Language detection does not equal cultural adaptation. A Spanish-speaking caller in Mexico may have different expectations than one in Spain.