Properties files - Lingo.dev CLI

What are Properties files?

Properties files are simple key-value text files commonly used in Java applications for configuration and localization.

For example:

app.title=MyApp
app.description=A simple demo application

user.greeting=Hello, world!
user.farewell=Thanks for using MyApp

error.message=Something went wrong
error.notFound=Page not found

What is Lingo.dev CLI?

Lingo.dev CLI is a free, open-source CLI for translating apps and content with AI. It's designed to replace traditional translation management software while integrating with existing pipelines.

To learn more, see Overview.

About this guide

This guide explains how to translate Properties files with Lingo.dev CLI.

You'll learn how to:

Create a project from scratch
Configure a translation pipeline
Generate translations with AI

Prerequisites

To use Lingo.dev CLI, ensure that Node.js v18+ is installed:

❯ node -v
v22.17.0

Step 1. Set up a project

In your project's directory, create an i18n.json file:

{
  "$schema": "https://lingo.dev/schema/i18n.json",
  "version": "1.10",
  "locale": {
    "source": "en",
    "targets": ["es"]
  },
  "buckets": {}
}

This file defines the behavior of the translation pipeline, including what languages to translate between and where the localizable content exists on the file system.

To learn more about the available properties, see i18n.json.

Step 2. Configure the source locale

The source locale is the original language and region that your content was written in. To configure the source locale, set the locale.source property in the i18n.json file:

{
  "$schema": "https://lingo.dev/schema/i18n.json",
  "version": "1.10",
  "locale": {
    "source": "en",
    "targets": ["es"]
  },
  "buckets": {}
}

The source locale must be provided as a BCP 47 language tag.

For the complete list of the locale codes that Lingo.dev CLI supports, see Supported locale codes.

Step 3. Configure the target locales

The target locales are the languages and regions you want to translate your content into. To configure the target locales, set the locale.targets property in the i18n.json file:

{
  "$schema": "https://lingo.dev/schema/i18n.json",
  "version": "1.10",
  "locale": {
    "source": "en",
    "targets": ["es"]
  },
  "buckets": {}
}

Step 4. Create the source content

If you haven't already, create one or more Properties files that contain the content to be translated. These files must be located at a path that includes the source locale somewhere in the path (e.g., as a directory name like en/ or as part of the filename like messages.en.properties).

Step 5. Create a bucket

In the i18n.json file, add a "properties" object to the buckets object:

{
  "$schema": "https://lingo.dev/schema/i18n.json",
  "version": "1.10",
  "locale": {
    "source": "en",
    "targets": ["es"]
  },
  "buckets": {
    "properties": {}
  }
}

In the "properties" object, define an array of one or more include patterns:
```
{
  "$schema": "https://lingo.dev/schema/i18n.json",
  "version": "1.10",
  "locale": {
    "source": "en",
    "targets": ["es"]
  },
  "buckets": {
    "properties": {
      "include": ["./[locale]/example.properties"]
    }
  }
}
```
These patterns define which files to translate.

The patterns themselves:
- must contain [locale] as a placeholder for the configured locale
- can point to file paths (e.g., "[locale]/messages.properties")
- can use asterisks as wildcard placeholders (e.g., "[locale]/*.properties")
Recursive glob patterns (e.g., **/*.properties) are not supported.

Step 6. Configure an LLM

Lingo.dev CLI uses large language models (LLMs) to translate content with AI. To use one of these models, you need an API key from a supported provider.

To get up and running as quickly as possible, we recommend using Lingo.dev Engine — our own, hosted platform that offers 10,000 tokens of free, monthly usage:

Sign up for a Lingo.dev account.
Run the following command:
```
npx lingo.dev@latest login
```
This will open your default browser and ask you to authenticte.
Follow the prompts.

Step 7. Generate the translations

In the directory that contains the i18n.json file, run the following command:

npx lingo.dev@latest run

This command:

Reads the i18n.json file.
Finds the files that need to be translated.
Extracts the translatable content from the files.
Uses the configured LLM to translate the extracted content.
Writes the translated content back to the file system.

The first time translations are generated, an i18n.lock file is created. This file keeps track of what content has been translated, preventing unnecessary retranslations on subsequent runs.

Example

en/example.properties

app.title=MyApp
app.description=A simple demo application

user.greeting=Hello, world!
user.farewell=Thanks for using MyApp

error.message=Something went wrong
error.notFound=Page not found

database.host=localhost
database.port=5432

notification.success=Changes saved!
notification.warning=Please check your input

es/example.properties

app.title=MyApp
app.description=Una aplicación de demostración simple

user.greeting=¡Hola, mundo!
user.farewell=Gracias por usar MyApp

error.message=Algo salió mal
error.notFound=Página no encontrada

database.host=localhost
database.port=5432

notification.success=¡Cambios guardados!
notification.warning=Por favor verifica tu entrada

i18n.json

{
  "version": "1.10",
  "locale": {
    "source": "en",
    "targets": ["es"]
  },
  "buckets": {
    "properties": {
      "include": ["./[locale]/example.properties"]
    }
  },
  "$schema": "https://lingo.dev/schema/i18n.json"
}

i18n.lock

version: 1
checksums:
  2250baa513dddb2c691cb62115d5e257:
    app.title: 7dc70110429d46e3685f385bd2cc941c
    app.description: e13baa1e885129d9328e216ff534761b
    user.greeting: 0468579ef2fbc83c9d520c2f2f1c5059
    user.farewell: 118794a2b84f7bfb4b4ce602ed463b0f
    error.message: a3cd2f01c073f1f5ff436d4b132d39cf
    error.notFound: 97612e6230bc7a1ebd99380bf561b732
    database.host: da86e4fc0c04d82c87006dc71cea7e97
    notification.success: 3b7a8b0aa23977592d4270ea136a390c
    notification.warning: c38895f731311cefacee9e8d7d10fc49