Multilingual Calls and Language Detection

A voice agent can run a call in one language, offer several and switch when asked, or work out which language the caller is speaking and follow them without being told. All three are configured from the Setup and Voice tabs.

What you'll need

The agents:voice:manage permission.
A saved voice agent — see Create a Voice Agent.

One language (the default)

On the Setup tab, Default Language sets the language the agent starts in and the language speech recognition is tuned for.

Six languages can be a Default Language: English, Spanish, French, German, Italian and Portuguese.

Hindi and Punjabi appear in the same list but are supported as additional languages rather than as the default — set the default to one of the six and add Hindi or Punjabi under Additional Languages (below). Choosing either as the Default Language is refused when the agent is saved.

Being a valid Default Language is not the same as having a voice built for it. The table under What is supported, language by language has the full picture; the short version is that English, French and Spanish are covered end to end, while German, Italian and Portuguese need you to choose the voice with care.

Leave Multi-Lingual Support off and you are done; the agent speaks one language in one voice.

Several languages, switched on request

flowchart TB
    A["Multi-Lingual Support: on"] --> B["Default Language<br/>+ primary voice"]
    A --> C["Additional Languages<br/>one voice per language"]
    B --> D["Agent starts in the default"]
    C --> E["Caller asks in, or asks for,<br/>another language"]
    E --> F["Agent switches language<br/>and swaps to that language's voice"]
    G["Auto-Detect Language: on"] --> H["Every turn is recognised<br/>language-agnostically"]
    H --> F

Open the Setup tab and turn on Multi-Lingual Support ("Configure different voices for different languages").
Go to the Voice tab. The voice picker's label changes to Primary Voice — that is the voice for your default language — and an Additional Languages card appears beneath it.
Click Add Language, then for each row:
- Language — pick from the same list.
- Voice — pick a voice for that language. Each option shows its engine, locale and gender, e.g. Daniella (Cartesia · ES · Female).
- Use the play button beside the row to preview it with your Preview Text, and the trash icon to remove the row.
Save Changes.

Turning on multilingual support also gives the agent its language-switching ability: it can move between the languages you configured when a caller asks, or when it hears the caller has changed. Each switch swaps the voice to that language's voice, so the accent follows the language rather than staying in the default's.

What is supported, language by language

Three things have to line up for a language, and they do not line up the same way for all eight in the picker: whether it can be the Default Language, whether speech recognition is set to it, and whether a voice tuned for it exists.

Language	As Default Language	Dedicated voices	What you get
English (`en`)	Yes	16 (incl. US and UK) + 10 emotive	Full support
French (`fr`)	Yes	8	Full support
Spanish (`es`)	Yes	2	Full support
German (`de`)	Yes	none	Works, but in a voice not tuned for German — see below
Italian (`it`)	Yes	none	Same
Portuguese (`pt`)	Yes	none	Same
Hindi (`hi`)	No — use it as an additional language	4	Full support as an additional language
Punjabi (`pa`)	No — use it as an additional language	2	Full support as an additional language

Plus 10 OpenAI Realtime voices (Alloy, Ash, Ballad, Coral, Echo, Fable, Onyx, Nova, Sage, Shimmer) which carry no language tag of their own and speak whichever language the call is set to.

German, Italian and Portuguese: supported, but pick the voice deliberately

All three save as the Default Language and speech recognition is set to them, so the agent understands callers fine. What is missing is a voice built for the language: the picker has none, so the agent speaks through whichever voice you selected.

That leaves two sensible choices:

An OpenAI Realtime voice — these are language-agnostic, so they speak German, Italian or Portuguese without carrying an English accent across. This is usually the better pick, and these are the only voices that also read your Voice-Specific Instructions.
A Cartesia voice — it will speak the language, but in that voice's own accent, because the voice was built for English or French. Preview it before you commit: type a sentence in the target language into Preview Text and listen.

Either way, preview in the actual language rather than the default English phrase. That is the only way to hear what your callers will hear.

Detect the language automatically

On the Setup tab, Auto-Detect Language ("Automatically detect the user's language each turn and switch voices on the fly") changes how speech recognition runs: instead of listening for one specific language, the agent recognises each turn language-agnostically and switches to the caller's language as it hears it.

Use it when:

Callers arrive on one number from several language communities, and asking them to choose is friction you would rather not add.
A caller code-switches mid-sentence — common in bilingual households and worth handling gracefully.

Leave it off when:

Everyone calls in one language. A single-language model is the more accurate choice, and there is nothing to gain from generality.
Your callers use heavy domain vocabulary, brand names or part numbers in one language. Recognition tuned to that one language handles them better.

Auto-detect pairs with Multi-Lingual Support: detection decides what the caller said, and the configured voices decide how the agent answers. Turn on detection without configuring an additional voice for a language and the agent may answer in a language it has no dedicated voice for, in the primary voice.

Writing instructions for a multilingual agent

Write them in one language — English is fine. The model works from them and speaks whatever the call is in; you do not need a translated copy per language.
Say what should not be translated: product names, legal wording, a quoted price format, the business name.
Say what to do when the caller's language is not configured. Offer a transfer, or continue in the default language and say so — either is better than silently answering in a language the caller did not use.
Keep the greeting language-neutral, or in the default. The greeting is spoken before the agent has heard anything, so detection has not run yet.

Troubleshooting

Saving fails after I picked Hindi or Punjabi as the Default Language. Those two are supported as additional languages. Set the default to English, Spanish, French, German, Italian or Portuguese, and add Hindi or Punjabi under Additional Languages.
The agent speaks German (or Italian, or Portuguese) with an English accent. No voice in the picker is built for those three, so it is using the one you selected. Switch to an OpenAI Realtime voice, which is language-agnostic, and preview it with a sentence in the target language.
The Additional Languages card is not there. Multi-Lingual Support is off, on the Setup tab.
The agent switches language but keeps the original accent. No voice is configured for the language it switched to. Add a row for it under Additional Languages.
It answers in the wrong language on the first turn. The greeting and the opening turn run in the default language — detection needs to hear the caller first. Set the default to your most common language.
Recognition got worse after turning on auto-detect. Expected on a single-language line: a language-agnostic model trades some accuracy for coverage. Turn it off if every caller speaks the same language.