How to configure stable multilingual agents (IT/EN/ES/FR/DE) with 2–3 languages per agent?

Support Help
1

**Hi Retell team,

I’m building a SaaS on top of Retell AI for restaurants in Europe and I need clear best practices for **multilingual voice agents**.

Languages I need to support overall:

  • Italian, English, Spanish, French, German

Product requirement:

  • For each restaurant I want to configure agents that are either:
    • **Monolingual** (e.g. only Italian), or
    • **Multilingual**, but limited to **2–3 languages per agent** (e.g. IT+ES, IT+EN+FR) on the same phone number.

What I observe now:

  • When I set up an Italian agent and start speaking Spanish, the transcription sometimes drifts and the agent eventually “normalizes” everything to Italian and replies in Italian.
  • I would like the agent to reliably:
    • detect which of the configured languages the caller is using,
    • always answer in that language,
    • and switch language only when the caller clearly switches (while keeping context).

Questions:

  1. **Model / language settings**

    • For a true multilingual agent (e.g. IT+ES or IT+EN+FR), what is the recommended way to configure the agent?
    • Should I use a specific **multilingual ASR/model setting**, and how should I set the `language` parameter in this case?
    • Any constraints when mixing these languages (IT/EN/ES/FR/DE) in the same agent?

  2. **Prompt design**

    • Is it recommended to explicitly list the supported languages in the prompt (e.g. “you are fluent in Italian and Spanish and only these languages”) and limit to max 2–3?
    • Do you have an **official example prompt** for multilingual agents that you recommend we use as a template?

  3. **Design pattern**

    • From your experience, is it better to:
      • use **one multilingual agent** per number (IT+ES, IT+EN+FR, etc.), or
      • create separate **monolingual agents** and route calls (e.g. language selection at the beginning of the call)?

Context:

  • Use case: restaurant reservations and information for Italian restaurants with many foreign customers (tourists).

Any detailed guidance, docs or examples specifically about configuring **stable multilingual voice agents** (not just the language list) would be very helpful, as I want to implement this correctly in my own platform.

Thank you!**

2

Hi @insegne.video

  1. Set the language parameter to multi for multilingual agents. This enables auto-detection and code-switching between languages. For more details you can see (Set language for your agent - Retell AI). The language setting controls ASR (speech-to-text) and influences TTS pronunciation, but it does not enforce response language — you must do that in the prompt.

  2. For Prompt Design, Yes, explicitly instruct the agent in the prompt to only respond in the configured languages, You have to set that in the prompt yourself. This can be as easy as adding something like ‘always respond in Spanish’ into the prompt.

  3. Text normalization (normalize_for_speech) currently only supports English, Spanish, French, and German. Italian is not supported for normalization, so numbers/dates may occasionally be read incorrectly in Italian. For more details you can see (Normalize text for speech - Retell AI).

  4. The multi option exists for code-switching, a single multilingual agent per number is a supported approach. Limiting to 2–3 languages via prompt instructions is a reasonable strategy to reduce ASR drift.

  5. All your languages (IT/EN/ES/FR/DE) are supported by ElevenLabs, Cartesia, and MiniMax. Choose a voice that sounds natural in your primary language.

Thank You