# normalizeLanguage

**Kind:** Function

**Source:** [`src/middleware/language/language.ts`](https://github.com/honojs/hono/blob/main/src/middleware/language/language.ts#L84)

**Part of:** [Middleware](subsystem-src-middleware)

Validate and normalize language codes

`normalizeLanguage` validates an incoming language code and converts it to the format expected by the language middleware. It acts as a boundary between external request values and the normalized language value used by the application.

## Signature

```ts
function normalizeLanguage(lang: string | null | undefined, options: DetectorOptions): string | undefined
```

## Parameters

| Name | Type |
|---|---|
| `lang` | `string | null | undefined` |
| `options` | `DetectorOptions` |

**Returns:** `string | undefined`

## Diagram

```mermaid
graph LR
  A[Incoming language code] --> B[normalizeLanguage]
  B --> C{Valid code?}
  C -->|Yes| D[Normalized language code]
  C -->|No| E[Fallback or validation result]
```

## Usage

```ts
import { normalizeLanguage } from './middleware/language/language';

const requestedLanguage = 'EN-us';
const language = normalizeLanguage(requestedLanguage);

console.log(language);
```

## AI Coding Instructions

- Pass request-derived language values through `normalizeLanguage` before using them for locale selection or translations.
- Keep language-code formatting rules in this function rather than duplicating case conversion or validation in middleware callers.
- Preserve the function’s handling of invalid or missing values when changing language middleware behavior.
- Update callers and tests together if supported language codes or fallback behavior changes.