Is LanguageTool really open source?

Partly. The core correction engine is published under the LGPL 2.1-or-later license on GitHub, and anyone can install it on their own server. But the AI-based rules, which make up most of the premium service's added value, stay proprietary and only run on LanguageTool's cloud API.

Do my texts train LanguageTool's AI?

No, according to its privacy policy, which states explicitly that texts submitted by users, free or paying, aren't used to train its algorithms or language models. That's a written commitment, clearer on this specific point than several competitors in this comparison.

Can you self-host LanguageTool to keep documents in-house?

Yes for the core engine, via the HTTP server documented by LanguageTool: the texts then never leave the user's infrastructure. The price to pay is functional: the AI-based rules, which cut false positives the most, are only available on the cloud API, not on this self-hosted server.

What's the difference with Grammarly or Antidote?

LanguageTool covers more than 30 languages and offers an open core engine, which neither Grammarly nor Antidote does. Antidote processes its core correction locally on the machine, without depending on a server; Grammarly and LanguageTool are primarily online services, one focused on professional English, the other on multilingual coverage.

LanguageTool: our review

LanguageTool corrects grammar and style in more than 30 languages, with an open-source core born in Germany. Since 2023, the service's legal publisher has been the American company Learneo — a fact its own name doesn't give away.

Free version, then paid

Go to LanguageTool's website

What we take away from it

What counts in its favor

The core engine (the "languagetool" repository) is published under the LGPL 2.1-or-later license on GitHub: a technical team can audit it and integrate it into its own code.
A self-hostable server exists, documented by LanguageTool, for those who want to keep basic correction on their own infrastructure.
The privacy policy states it outright: "Learneo does not use inputs from any LanguageTool user […] to train its language algorithms or models."

What counts against it

The legal publisher, Learneo Inc., is based in San Francisco and incorporated in Delaware: despite its origins and an office in Germany, it's no longer, under GDPR, a company controlled from the EU.
The self-hosted server doesn't include the AI-based rules, reserved for the cloud API according to LanguageTool's technical documentation: self-hosting protects privacy but degrades correction quality.
Premium and Team pricing didn't display reliably on the site at the time of checking ("There was an issue displaying the prices"), which prevented a precise cost comparison with the other correctors in this comparison.

The most technically open choice in this comparison, provided you don't trust its name for data location: it has been an American product since the Learneo acquisition, with a self-hostable core engine that loses its AI rules once taken out of the cloud.

The facts

Data hosting: Hosting not verified
Categories: Proofreading and rewriting
Data verified on: 30 July 2026

What happens to your data

Is your data used to train their models?: No
Subprocessors: The legal publisher is Learneo, Inc., with offices in the United States, India, the Netherlands, Germany and Canada according to its privacy policy. Transfers outside the European Economic Area rely on the EU-U.S. Data Privacy Framework and the European Commission's standard contractual clauses. The exact physical location of the servers isn't published.
Retention: Personal data is kept, according to the privacy policy, "for as long as the account remains open, and for a reasonable period necessary for the purposes described" — with no numeric duration beyond that wording.
Data processing agreement: Document published by the publisher

LanguageTool in detail

LanguageTool corrects grammar, spelling, punctuation and style in more than thirty languages, through a browser extension, an office add-in or an API. The project started in 2003 as free software in Potsdam, Germany, before becoming a company, then being acquired in 2023 by the American firm Learneo — which, incidentally, also owns Quillbot, already listed in this comparison.

What it actually does

The browser extension underlines errors directly in Gmail, a form or a Google document, with an explanation of the rule broken rather than a silent correction. The Word and LibreOffice add-in does the same thing partially offline, with the free version limited to 2,000 characters per check — compare that with the other free AI tools in the directory. The Premium version extends this limit to 150,000 characters and adds extra rules that catch awkward phrasing a purely grammatical checker would let through. For a technical team, the documented HTTP API also allows installing a LanguageTool server on your own infrastructure, without relying on the online service. The Team and Business tiers add a shared dashboard and a company glossary, to fix the terminology for a given sector rather than let each writer choose their own terms.

Why it interests us in particular

The product's name and its German history mask a fact our clients need to know before deploying it: since 2023, the entity that legally operates LanguageTool is Learneo, Inc., an American company based in San Francisco and incorporated in Delaware. The site keeps an office in Germany, but it's no longer a company controlled from the EU in the sense the GDPR generally uses. Conversely, LanguageTool documents a genuine way out for anyone who wants to stay in control: the core engine, published under the LGPL licence, can be self-hosted, which may interest an internal IT department as part of our audit and advisory engagements.

Where it falls short

Self-hosting has a functional cost: the AI-based rules, which cut false alerts the most and improve rewordings, only run on LanguageTool's cloud API, not on the server you install yourself. Keeping control of the infrastructure, here, means giving up the best version of the product. A second, more mundane caveat: at the time of our check, the Premium pricing page showed a price-loading error, making it impossible to run a numeric comparison against the other checkers in this comparison. Finally, the exact physical location of the servers is documented nowhere; only the countries where Learneo has offices are.

Our recommendation

LanguageTool suits anyone who prioritises language coverage and the technical option of self-hosting, along with a written commitment not to train its models on submitted text. For a firm that requires a legally European host, the current vendor's American nationality disqualifies the tool, whatever its name might suggest.

Who it's for, and who it isn't

Suited for

Technical teams who want to audit or self-host the core correction engine, under the LGPL license
Multilingual users who correct in more than thirty languages and want a single tool for all of them
Organizations satisfied with a written commitment not to train on their texts, without requiring a European host

Not a fit

Firms or SMEs whose policy requires a publisher whose headquarters and servers stay in the EU: Learneo, the company that operates LanguageTool, is American
Teams that want to self-host without losing anything: the AI-based rules only work on the cloud API, not on the self-hosted server
Buyers who need to compare a precise cost before choosing: the Premium pricing didn't display correctly during our check

Frequently asked questions

Is LanguageTool really open source?: Partly. The core correction engine is published under the LGPL 2.1-or-later license on GitHub, and anyone can install it on their own server. But the AI-based rules, which make up most of the premium service's added value, stay proprietary and only run on LanguageTool's cloud API.
Do my texts train LanguageTool's AI?: No, according to its privacy policy, which states explicitly that texts submitted by users, free or paying, aren't used to train its algorithms or language models. That's a written commitment, clearer on this specific point than several competitors in this comparison.
Is LanguageTool a German company?: It used to be: the project was born in Potsdam in 2003, later developed by the German company LanguageTooler GmbH. Since its 2023 acquisition, the entity that legally operates LanguageTool is Learneo, Inc., an American company based in San Francisco and incorporated in Delaware, still with an office in Germany according to its privacy policy.
Can you self-host LanguageTool to keep documents in-house?: Yes for the core engine, via the HTTP server documented by LanguageTool: the texts then never leave the user's infrastructure. The price to pay is functional: the AI-based rules, which cut false positives the most, are only available on the cloud API, not on this self-hosted server.
What's the difference with Grammarly or Antidote?: LanguageTool covers more than 30 languages and offers an open core engine, which neither Grammarly nor Antidote does. Antidote processes its core correction locally on the machine, without depending on a server; Grammarly and LanguageTool are primarily online services, one focused on professional English, the other on multilingual coverage.