# gt-next: General Translation Next.js SDK: isLocaleSupported
URL: https://generaltranslation.com/en-US/docs/next/api/helpers/is-locale-supported.mdx
---
title: isLocaleSupported
description: API reference for the isLocaleSupported server-side method
---

## Overview

The `isLocaleSupported` function checks whether a locale is valid and supported by your app's current gt-next configuration.

Use it when a route param or other request input should be explicitly rejected instead of silently falling back to the default locale. A common pattern is validating the `[locale]` segment in a dynamic route and calling `notFound()` when the value is not supported.

<Callout>
  `isLocaleSupported` is a server-side method and can only be used on
  server-side components.
</Callout>

## Reference

### Parameters

<TypeTable
  type={{
    "locale": {
        type: 'unknown',
        optional: false,
    },
  }}
/>

| Name     | Description                                                                                              |
|----------|----------------------------------------------------------------------------------------------------------|
| `locale` | The locale candidate to validate. Any value is accepted; non-string or invalid BCP 47 values return `false`. |

### Returns

A `boolean` that is `true` when the locale resolves to one of the configured locales, and `false` otherwise.

It also acts as a TypeScript type guard (`locale is string`), narrowing the value to `string` when it returns `true`.

---

## Examples

### Rejecting invalid route params

Validate the `[locale]` route param and call `notFound()` for unsupported locales instead of relying on the default-locale fallback.

```tsx title="app/[locale]/page.tsx" copy
import { notFound } from 'next/navigation';
import { isLocaleSupported } from 'gt-next/server';

export default async function Page({
  params,
}: {
  params: Promise<{ locale: string }>;
}) {
  const { locale } = await params;

  if (!isLocaleSupported(locale)) {
    notFound(); // [!code highlight]
  }

  return <p>Locale: {locale}</p>;
}
```

---

## Notes

- `isLocaleSupported` is server-side only.
- Without this check, an invalid or unsupported request locale falls back to the default locale rather than throwing. See [`getLocale`](/docs/next/api/helpers/get-locale) for the fallback behavior.
- The set of supported locales is configured in [`withGTConfig`](/docs/next/api/config/with-gt-config). See also [`gt.config.json`](/docs/next/api/config/gt-config-json).

## Next steps

- See [`getLocale`](/docs/next/api/helpers/get-locale) to retrieve the user's current locale.
- Learn how to configure supported locales with [`withGTConfig()`](/docs/next/api/config/with-gt-config).