Translate Java .properties files

A .properties file is a bundle of key=value lines where the values hold {0} arguments for MessageFormat. KAERIS translates the values, keeps your comments and key order, and writes the file in pure ASCII so Java reads it everywhere.

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How a .properties file is built

key=value, key: value or key value, comments with # or !, line continuation with a trailing backslash. Older Java versions read bundles as ISO-8859-1, so non-Latin text is written as \uXXXX, which every version reads.

Source filemessages.properties

# Login form
login.title=Sign in to {0}
login.attempts=You have {0} attempts left
cart.total=Total: {0,number,currency}
about.url=https\://example.com/about
help.text=Need help?\nWrite to [email protected]

What to watch for in .properties translations

MessageFormat arguments
{0}, {1} and typed ones like {0,number,currency} must come back whole. In the real run below {0,number,currency} came back typed; a changed or lost argument would be reported.
Encoding
Output is ASCII: a German “ö” or a Japanese character is written as a \u escape, which every Java version reads.
Quotes in MessageFormat
In Java's MessageFormat a single quote starts a quoted section, so a pattern that uses arguments needs '' for an apostrophe. Check strings with apostrophes in the app after translating.
Escapes and URLs
\n in a value stays a line break. A \: in a URL is written back as a plain colon, which is also valid.

A real run: translated to German and checked

A real run, 27 October 2026, nothing edited by hand: kaeris translate messages.properties --langs de with kaeris 0.2.35, 3.3 seconds, model openai/gpt-4.1-mini (the service's fallback model answered this job). Below, the German file it wrote and what kaeris check says about it.

German resultde/messages.properties

# Generated by KAERIS i18n Tool - de
# Login form
login.title=Anmelden bei {0}
login.attempts=Sie haben noch {0} Versuche
cart.total=Gesamt: {0,number,currency}
about.url=https://example.com/about
help.text=Brauchen Sie Hilfe?\nSchreiben Sie an [email protected]

kaeris checkexit code 0

$ kaeris check --source messages.properties --langs de
✓ de: complete & placeholder-safe
✓ all locales complete & placeholder-safe

What KAERIS does with it

Comments and order kept
Comment lines, blank lines and the key order are re-emitted around the translated values, and a one-line header with the language is added.
Arguments counted
A lost {0} or a changed type is found without a model.
Free check for this format
The free check on /check reads this format and runs without a key or an account. The terminal command kaeris check reads it too: it takes all 13 formats since kaeris 0.2.35, with the same rules and nothing sent anywhere.

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Run it in three steps

Install the CLI
Run pip install kaeris.
Translate the bundle
Run kaeris translate messages.properties --langs de,ja.
Check the result
Upload the source and the translated bundle on the free check page.

pip install kaeris
kaeris translate src/main/resources/messages.properties --langs de,ja --verify

Questions

Does it write UTF-8 or \u escapes?

Escapes. The output file is ASCII, with non-ASCII characters written as \uXXXX.

Are my comments kept?

Yes. Comment and blank lines of the source are written back in place, and a header line naming the language is added.

Is it free?

Yes, without an account: 10,000 characters per file and up to 8 languages per run. A free API key raises the file limit to 50,000 characters. Lifetime is $79 once, on your own OpenRouter key; monthly plans start at $19. Pricing

Can I run it from the terminal or in CI?

Yes. pip install kaeris, then kaeris translate messages.properties --langs de,ja. The GitHub Action runs the same translation in a pull request. CLI docs · GitHub Action

Keep going

CLI reference · Check reference · Translate in the browser · Free check · Convert formats · Pseudo-localization

Other formats: JSON · YAML · .po · ARB · XLIFF · .strings · .xcstrings · .resx · CSV · Markdown · Fluent · Android XML