> ## Documentation Index
> Fetch the complete documentation index at: https://docs.poly.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Languages

<p className="lead">
  Languages configure which languages your agent supports. A project has one default language and zero or more additional languages.
</p>

Language configuration drives translation validation — every configured language must have an entry in each translation key.

## Location

Languages are defined in:

```text theme={"theme":{"light":"github-light","dark":"github-dark"}}
agent_settings/languages.yaml
```

The file is optional. When absent, the project runs as a single-language agent in the platform default.

## What languages contains

| Field                  | Description                                                |
| ---------------------- | ---------------------------------------------------------- |
| `default_language`     | The primary language code in BCP 47 format (e.g. `en-GB`). |
| `additional_languages` | List of additional language codes the agent supports.      |

## Example

```yaml theme={"theme":{"light":"github-light","dark":"github-dark"}}
default_language: en-GB
additional_languages:
  - fr-FR
  - de-DE
```

## Validation

* `default_language` is required and must be a valid BCP 47 language tag.
* Each entry in `additional_languages` must be a valid BCP 47 language tag.
* A language code cannot appear as both the default and an additional language.
* Duplicate additional language codes are not allowed.

<Tip>
  **Use region subtags**

  Use standard BCP 47 codes with region subtags (e.g. `en-GB`, `fr-FR`, `de-DE`) so downstream voice and TTS configuration can resolve unambiguously.
</Tip>

## Best practices

* set the default language to the primary language of your user base
* add additional languages only when translations are ready for every translation key
* keep BCP 47 codes consistent across `languages.yaml`, `translations.yaml`, and voice settings

## Related pages

<CardGroup>
  <Card title="Translations" href="/adk/reference/translations">
    Define the localized text strings used for each configured language.
  </Card>

  <Card title="Agent settings" href="/adk/reference/agent_settings">
    See how `languages.yaml` fits alongside personality, role, and rules.
  </Card>

  <Card title="Voice settings" href="/adk/reference/voice_settings">
    Configure per-language voice and TTS behaviour for multilingual agents.
  </Card>
</CardGroup>