feat: add localization support to MCP resource operations

[stevenceuppens]

What?

Adds full localization support to the MCP plugin's resource operations (create, update, find, delete).

Why?

The MCP plugin did not expose Payload's localization capabilities. Users with multilingual content could not create or manage translations through MCP, limiting the plugin's usefulness for international projects. This brings MCP feature parity with Payload's REST API.

How?

• Add locale and fallbackLocale parameters to all resource tools (create, update, find, delete)
• Pass locale parameters through to underlying Payload operations
• Add comprehensive integration tests for localization features (6 new tests, all passing)
• Update documentation with localization usage examples and MCP client configuration
• Follow Payload REST API localization pattern for consistency

Testing:

pnpm test:int plugin-mcp
# All 15 tests passing (9 existing + 6 new localization tests)

Example Usage:

// Create content in English
{ "name": "createPosts", "arguments": { "title": "Hello", "locale": "en" }}

// Add Spanish translation
{ "name": "updatePosts", "arguments": { "id": "123", "title": "Hola", "locale": "es" }}

// Retrieve all translations
{ "name": "findPosts", "arguments": { "id": "123", "locale": "all" }}

Fixes #

[Copilot]

Pull Request Overview

This PR adds comprehensive localization support to the MCP plugin, enabling multilingual content management through MCP that matches Payload's REST API capabilities.

Key Changes:

• Added locale and fallbackLocale parameters to all resource operations (create, update, find, delete)
• Implemented 6 new integration tests covering localization scenarios (all passing)
• Updated documentation with localization usage examples and MCP client setup guide

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/plugin-mcp/int.spec.ts	Added 6 comprehensive localization tests and updated getApiKey helper to support update/delete permissions
test/plugin-mcp/config.ts	Configured localization with en/es/fr locales and enabled update/delete operations for Posts collection
test/plugin-mcp/collections/Posts.ts	Marked title and content fields as localized
packages/plugin-mcp/src/mcp/tools/schemas.ts	Added locale and fallbackLocale parameters to find, create, update, and delete tool schemas
packages/plugin-mcp/src/mcp/tools/resource/update.ts	Implemented locale parameter handling in update operation and added to schema
packages/plugin-mcp/src/mcp/tools/resource/find.ts	Implemented locale parameter handling in find operation
packages/plugin-mcp/src/mcp/tools/resource/delete.ts	Implemented locale parameter handling in delete operation
packages/plugin-mcp/src/mcp/tools/resource/create.ts	Implemented locale parameter handling in create operation with schema restructuring
docs/plugins/mcp.mdx	Added comprehensive documentation for MCP client setup and localization features with examples

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[kendelljoseph]

This is a great start @stevenceuppens!

I'll dig into more detail today.

[DanRibbens]

Looks like there are test failures need to be addressed https://github.com/payloadcms/payload/actions/runs/18868951904/job/53872817402?pr=14334#step:12:13889