Grammar police, open-source and multilingual

What it does

LanguageTool is an open-source proofreading engine written in Java that checks grammar and style across more than 25 languages. It goes beyond simple spell checking to catch errors that dictionaries alone won’t flag. The project ships as a standalone tool, a server with an HTTP API, and a LibreOffice/OpenOffice extension.

The interesting bit

The real work happens in community-contributed error detection rules, not just machine learning black boxes. The project maintains a public server, but the core is LGPL-licensed and designed to run self-hosted — useful if you’re checking text you’d rather not ship to a third party.

Key highlights

• Covers English, Spanish, French, German, Portuguese, Polish, Dutch, and 20+ others
• HTTP API with Swagger documentation for integration into editors or pipelines
• Multiple deployment shapes: GUI, command-line, server, or office suite plugin
• Self-hostable; community Dockerfiles available (though not officially maintained)
• Shallow clone drops build footprint from ~500 MiB download to under 60 MiB

Caveats

• Full source clone is hefty: over 500 MiB download, 1.5 GiB on disk
• Mac M1/M2 builds currently require Rosetta/x86_64 toolchain
• Bleeding edge builds may contain regressions

Verdict

Worth a look if you need offline-capable, multilingual text checking in a product or pipeline. Skip it if you’re after a lightweight drop-in library — this is a substantial Java project with Maven build overhead.