A translation extension reads every page you translate, and many of them hold your API keys too. That is a good reason to pick one whose code anyone can read. There are several open-source options now, and most of them put the translation under each paragraph of the original. The differences are in a few places:
- what it translates besides web pages: selected text, input boxes, video subtitles, documents;
- which translation engines it works with, and which of them need no API key;
- whether it helps you learn the words you look up;
- its licence, how it is paid for, and which browsers it runs in.
Below we go through six open-source extensions on those points. We make Read Frog, so it comes first. Everything about the others comes from their own websites, docs, store listings or GitHub repositories, checked in October 2026, when all six were still being updated. These extensions change often, so check each one's site before you install. If you are weighing Read Frog against Immersive Translate, which is not open source, see Read Frog vs Immersive Translate.
At a glance
| Extension | Bilingual pages | Also translates | Learning words | Licence |
|---|---|---|---|---|
| Read Frog | ✓ | Selection, input boxes, subtitles, PDFs | AI lookups, notes, FSRS review | GPL-3.0 |
| FluentRead | ✓ | Selection, input boxes, subtitles, images, documents | Save words, review, Anki export | GPL-3.0 |
| KISS Translator | ✓ | Selection, input boxes, YouTube subtitles | Save words | GPL-3.0 |
| OnlyTranslate | ✓ | Selection, input boxes, subtitles | — | GPL-3.0 |
| Interline | ✓ | Selection, input boxes | AI vocabulary notes | MIT |
| Linguist | Original on hover | Selection, Netflix subtitles | Personal dictionary | BSD-3-Clause |
"—" means neither the official site nor the docs mention it. Supported browsers are listed in each extension's section below.
1. Read Frog
An open-source extension that translates web pages side by side and helps you learn from what you read: look up a word, break down a long sentence, and review what you save as flashcards.
- Where it works: whole web pages, bilingual or translation only, either the main content or everything on the page; one paragraph at a time with hover translation; selected text; what you type in an input box, translated when you press space three times; subtitles on YouTube, X and Netflix. On readfrog.app you can also translate PDFs with their layout kept.
- Translation engines: Google Translate and Microsoft Translator work without an account or API key. You can add DeepL or DeepLX, an LLM such as OpenAI, DeepSeek, Gemini or Claude with your own key, any OpenAI-compatible API, or a local model through Ollama or LM Studio. With an LLM, AI Smart Context sends a summary of the page along with each paragraph, so names and terms come out the same way every time.
- Learning features: select a word or sentence on a page or in the subtitles and run a Custom AI Action, such as a dictionary lookup or Sentence Analysis. With a free account, save the result to your Notebase together with the sentence it came from, add a card template, and FSRS schedules each review.
- Open source: yes, GPL-3.0.
- Pricing: page, selection, input and subtitle translation are free on a provider of your own, with no account needed. Pro and Ultra add a built-in AI allowance for those features, unlimited notes and reviews, and AI subtitles for videos without captions.
- Browsers: Chrome, Edge, Firefox.
A good fit if you translate in order to learn the language and want the words you look up to come back for review. The settings are explained in Page and Paragraph Translation.
2. FluentRead
An open-source extension that covers a lot of ground: web pages, images, documents, and video and meeting captions, with an AI reading card and a learning center for the words you save.
- Where it works: web pages, bilingual or translation only; hover, selection and input box translation; text in images and in an area of the screen you select; PDF, ePub and Word files, which you can read in two languages, edit and export; bilingual subtitles on YouTube, X, Udemy and Disney+, and captions in the web versions of Google Meet, Teams and Zoom. For X videos without captions, the desktop extension can transcribe the audio on your device.
- Translation engines: free services that need no key, Google among them; DeepL and DeepLX; cloud translators such as Azure, Google Cloud, Alibaba Cloud, Tencent Cloud, Baidu and Volcengine; AI services and OpenAI-compatible endpoints with your own key; local models through Ollama, or small models you download into the extension. Each feature can use its own provider.
- Learning features: the AI reading card explains meaning, sentence structure and usage with the paragraph as context, and takes follow-up questions. Save words, phrases and sentences with their context to the learning center, review them by recalling each one and rating how well you knew it, or export them to Anki.
- Open source: yes, GPL-3.0.
- Pricing: free, with no FluentRead account; any paid service you connect is billed by its provider.
- Browsers: Chrome, Edge, Firefox, plus a userscript.
A good fit if you translate documents and images as well as web pages, and want a word list you can export to Anki.
3. KISS Translator
A minimalist open-source bilingual translator, also available as a userscript, that gives you close control over how each site is translated. Settings and site rules sync through WebDAV or a KISS-Worker you host yourself.
- Where it works: web pages, bilingual or translation only, using automatic text detection or rules you write for a site; selection and hover translation; input box translation with a shortcut; YouTube subtitles, with AI sentence segmentation and custom subtitle styles. A separate translation window compares several services side by side.
- Translation engines: Google and Microsoft; Tencent and Volcengine; DeepL and DeepLX; OpenAI, Gemini, Claude, DeepSeek, OpenRouter and Ollama; Azure AI, Cloudflare AI and Chrome's built-in AI translation; and any other API through a custom setup. You can keep an AI glossary of your own terms.
- Learning features: an English dictionary in the selection pop-up and a list of saved words, which are highlighted when they turn up on a page. Review is not mentioned.
- Open source: yes, GPL-3.0.
- Pricing: free; AI services are billed through your own key.
- Browsers: Chrome, Edge, Firefox, plus a Thunderbird add-on. The userscript runs in Tampermonkey or Violentmonkey, and in Safari on iOS through the Userscripts app.
A good fit if you want to tune how each site is translated and keep your sync on your own server.
4. OnlyTranslate
A newer extension built on FluentRead's code, with fewer settings and a narrower focus on translating what you read.
- Where it works: web pages, bilingual or translation only, either the main reading content or the whole page; selection, hover and input box translation; subtitles on YouTube, Udemy, Coursera and Khan Academy, when the video has its own subtitle track.
- Translation engines: Microsoft and Google with no setup, and Chrome's built-in translator, which runs in the browser; DeepL, OpenAI, DeepSeek, Gemini and Claude with your own key, and OpenAI-compatible gateways.
- Learning features: its site does not mention saving or reviewing words.
- Open source: yes, GPL-3.0.
- Pricing: free, with no account and no subscription; services you add are billed by their provider.
- Browsers: Chrome, Edge, Firefox.
A good fit if you want FluentRead's page translation with fewer options to set, plus subtitles on course sites.
5. Interline
A Chrome extension for reading and writing in another language: bilingual pages, selection lookup, and translating a draft where you type it.
- Where it works: web pages, shown bilingual, translation only or original only, in ten display styles; selected text, which you can read, copy or listen to; drafts in input boxes and several rich-text editors, translated in place when you press space three times. Site rules decide which sites translate automatically.
- Translation engines: a built-in Google Translate engine with no key; OpenAI, Anthropic, Gemini or OpenRouter with your own key, or any service compatible with the OpenAI or Anthropic API, including a local Ollama. With an AI engine you can set translation styles, prompts and a glossary.
- Learning features: supported AI engines add vocabulary notes to a selection translation. Saving words is not mentioned.
- Open source: yes, MIT.
- Pricing: free, with no account; AI providers bill you for API use.
- Browsers: Chrome 116 or later.
A good fit if you write in a foreign language as often as you read it, and you only use Chrome.
6. Linguist
An open-source translator built around privacy: it can translate on your device, without an internet connection.
- Where it works: whole web pages, translated in place: the translation replaces the text, and you hover over it to see the original. Selected text, text you type in its pop-up, and Netflix subtitles.
- Translation engines: Google, Microsoft and Yandex out of the box, plus an offline translator that runs on your device. Custom translator modules add services such as ChatGPT, DeepL, Gemini, Ollama or LibreTranslate.
- Learning features: every translation goes into a searchable history, and you can save entries to a personal dictionary that works offline. Text-to-speech reads translations aloud. Review is not mentioned.
- Open source: yes, BSD-3-Clause.
- Pricing: free, supported by donations.
- Browsers: Chrome and other Chromium browsers, Firefox, including Firefox for Android.
A good fit if you want translations that never leave your device, and you don't need the original beside the translation.
How to choose
- You just want to read foreign pages: all six translate whole pages and work without an API key. Five keep the original beside the translation; Linguist replaces the text and shows the original on hover.
- You want to learn the words you look up: Read Frog and FluentRead save words with their context and bring them back for review; Read Frog schedules reviews with FSRS, and FluentRead exports to Anki. KISS Translator and Linguist save words but don't mention review.
- You translate more than web pages: Read Frog, FluentRead, KISS Translator and OnlyTranslate translate video subtitles. FluentRead also handles images, documents and meeting captions, and Read Frog translates PDFs on readfrog.app.
- You want text to stay on your machine: Linguist translates offline on your device. Read Frog, FluentRead, KISS Translator and Interline can use a local model through Ollama.
- The licence matters to you: Read Frog, FluentRead, KISS Translator and OnlyTranslate are GPL-3.0, so anyone who distributes a modified version must publish its code too. Interline (MIT) and Linguist (BSD-3-Clause) are permissive licences that also allow closed-source reuse.
- You don't use Chrome: Interline is Chrome-only. The other five run in Firefox; KISS Translator and FluentRead also come as userscripts.
FAQ
Is Immersive Translate open source?
No. Its main GitHub repository carries no open-source licence; an early open-source version was archived in 2023. All six extensions above publish their full source code.
Do I need an API key?
Not to get started. All six come with a free engine, such as Google Translate or Microsoft Translator, that needs no key. You need your own key only for AI models like OpenAI, DeepSeek or Claude, and the provider bills you for what you use.
Can I install two of them at once?
Yes, but turn on page translation in only one of them on any given site, or each paragraph gets translated twice.