L10n.dev - AI Localization Agent

ai-l10n-mcp

[npm version] [License: MIT]

MCP server that gives AI agents access to l10n.dev — a professional localization service purpose-built for software i18n files.

When an AI agent needs to translate your app, the naive approach is to paste file contents into the chat. That breaks down fast: localization files are large, formats are strict, placeholders must survive verbatim, glossaries drift across sessions, and the agent has no memory between runs. This MCP replaces that fragile workflow with a dedicated localization engine that the agent calls as a tool — getting professional-grade output without wasting context window on raw file contents.

Think of it as giving your AI agent a professional localization co-pilot: the agent handles intent and orchestration; l10n.dev handles the actual translation with accuracy, consistency, and format guarantees.

Why use this instead of asking your AI agent directly?

Features

• 165 Languages — translate to the full range of world languages in one call
• Format guaranteed — JSON, JSONC, Flutter ARB, YAML, PO (gettext), XLIFF, MD and all text-based i18n formats; placeholders, keys, and structure preserved and validated after translation
• Auto-detect target languages from project structure
• Persistent glossary — generate a glossary from your content, save it, and have it applied automatically across every subsequent file and chunk for consistent terminology
• Linguistic instructions — save style/tone rules per language pair (e.g. "Use formal tone")
• Incremental translation — hash-based change detection skips strings already translated, saving quota and protecting existing translations
• Proactive quality control — checks instructions and glossary for each language pair before translating, not after

Connect Claude Desktop, Cursor, Windsurf, GitHub Copilot, OpenAI Codex, or any MCP-compatible agent directly to l10n.dev's translation engine.

Installation & Configuration

Getting an API key

Create a free account and get your API key at https://l10n.dev/ws/api-keys

Tip: Instead of setting L10N_API_KEY in each config, you can ask the AI to store your key with l10n_set_api_key. The key is saved to ~/.ai-l10n/config.json and used automatically.

Claude Desktop

In Claude Desktop, open Settings > Developer > Edit Config. Claude Desktop will open the correct MCP config file for your install. On macOS this is typically ~/Library/Application Support/Claude/claude_desktop_config.json. On Windows, the backing path can vary by installation, so prefer Edit Config instead of navigating manually. Add:

{
  "mcpServers": {
    "l10n": {
      "command": "npx",
      "args": ["-y", "ai-l10n-mcp"],
      "env": {
        "L10N_API_KEY": "your-api-key-here"
      }
    }
  }
}

Cursor

Open Customize in Cursor to add and manage MCP servers. For a file-based setup, create one of these configs:

• ~/.cursor/mcp.json for a user-wide setup
• .cursor/mcp.json in your project for a workspace-specific setup

Add:

{
  "mcpServers": {
    "l10n": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "ai-l10n-mcp"],
      "env": {
        "L10N_API_KEY": "your-api-key-here"
      }
    }
  }
}

Windsurf

In Windsurf, open the MCPs panel in Cascade, or go to Devin Settings > Cascade > MCP Servers. If you need to add it manually, edit ~/.codeium/windsurf/mcp_config.json and add:

{
  "mcpServers": {
    "l10n": {
      "command": "npx",
      "args": ["-y", "ai-l10n-mcp"],
      "env": {
        "L10N_API_KEY": "your-api-key-here"
      }
    }
  }
}

GitHub Copilot (VS Code)

Open the Command Palette and select MCP: Open User Configuration. Alternatively, create a .vscode/mcp.json file in your workspace or user settings. Add:

{
  "servers": {
    "l10n": {
      "type": "stdio",
      "command": "npx",
      "args": ["-y", "ai-l10n-mcp"],
      "env": {
        "L10N_API_KEY": "your-api-key-here"
      }
    }
  }
}

OpenAI Codex (CLI and IDE)

Add to ~/.codex/config.toml for a user-wide setup, or .codex/config.toml in a trusted project:

[mcp_servers.l10n]
command = "npx"
args = ["-y", "ai-l10n-mcp"]

[mcp_servers.l10n.env]
L10N_API_KEY = "your-api-key-here"

You can also add it from the terminal:

codex mcp add l10n --env L10N_API_KEY=your-api-key-here -- npx -y ai-l10n-mcp

Claude Code (CLI and VS Code extension)

Add the server from the terminal:

claude mcp add --env L10N_API_KEY=your-api-key-here --transport stdio l10n -- npx -y ai-l10n-mcp

For a shared project setup, Claude Code can also store MCP servers in a .mcp.json file at your project root.

Available Tools

Translation

Both tools detect language codes in language-code folders (locales/en/common.json, reported as folder-based) and in file names (file-based) — whether the name is the code itself (locales/en.json) or holds it alongside other parts (app_en.arb, locales/emails.en.json, locales/en-US.common.json, messages_en_US.properties). For naming conventions that are not recognized, pass languageCodeRegex, a regex containing a (?<language>...) group, e.g. ^emails\.(?<language>[\w-]+)\.json$. The text around the group is reused for target files, so emails.en.json produces emails.ru-RU.json.

Linguistic Instructions

Glossary

Account

Available Prompts

l10n_project_setup

Guides through checking and configuring linguistic instructions and glossaries for optimal translation quality. Invoke it at the start of a new project or when reviewing l10n.dev settings.

Arguments:

• sourceLanguage — source language code (default: en)
• targetLanguages — comma-separated target language codes (e.g. es,fr,de)

l10n_setup_automation

Scans the project for i18n source files (using l10n_detect_project_structure), checks linguistic instructions, glossary, and balance, then interactively sets up fully automated translation via GitHub Actions or npm scripts.

For GitHub Actions, guides through four trigger options:

• A) On every push to main — translate and commit back directly
• B) On pull requests — translate and open a PR for review
• C) On a schedule — translate nightly or on a cron
• D) Manual trigger only — workflow_dispatch

Writes ai-l10n.config.json and .github/workflows/translate.yml (or updates package.json scripts), then tells you where to add the API key and what happens on the next trigger.

Arguments:

• sourceLanguage — source language code (default: en)
• targetLanguages — comma-separated target language codes (e.g. es,fr,de)

Example Workflow

User: "Translate my app to Spanish and French"

Claude (with this MCP server) will:

1. Call l10n_list_instructions — finds no instruction for es/fr language pairs
2. Ask: "No instruction found for Spanish/French — would you like to set a tone/style rule before translating? (e.g. formal, casual, keep brand terms untranslated)"
3. User says: "Tone should be informal, it's for a food app in Latin America"
4. Call l10n_create_instruction with the style rule
5. Call l10n_list_glossaries — finds no active glossary for es/fr
6. Ask: "No glossary found for Spanish/French — enable glossary generation for this run? It saves key terms for consistent future translations."
7. User says: "Yes"
8. Detect that es.json and fr.json already exist — ask: "Target files already exist — enable incremental mode to skip unchanged strings and save quota?"
9. User says: "Yes"
10. Call l10n_translate_file with sourceFile, targetLanguages: ["es", "fr"], instruction, generateGlossary: true, translateOnlyNewStrings: true
11. Report results

Incremental Translation

For JSON-based formats, enable translateOnlyNewStrings: true to skip strings that are already translated. A hash of each source string (not content itself) is stored on the l10n.dev server for change detection — only added or changed strings are translating, saving your character quota.

Note: First translation it translates only added strings, because hash table is empty to detect changed strings.

Automation: CLI and GitHub Actions

For CI/CD automation and command-line usage, see:

• ai-l10n CLI — translate files from the terminal or CI pipelines
• GitHub Action — auto-translate on push, PR, or manual trigger

Pricing

• Free tier — 10,000 characters free every month, no credit card required.
• Pay-as-you-go — Affordable character-based pricing with no subscription required.
• Current packages — Visit l10n.dev/#pricing for up-to-date pricing.

Privacy & Security

• No data retention — Source text and translations are not stored on l10n.dev servers beyond the time needed to process the request.
• Encrypted communication — All API calls use HTTPS.
• Privacy first — Built by developers for developers, with privacy, reliability, and quality as core priorities.

Support

License

MIT