The same bilingual reading model — original paragraph, translation right under it, dual video subtitles — as free software with your own key. An honest comparison, including the parts where the other tools are better.
BelliedMonkey Translator is a free, GPL-3.0 open-source browser extension for Safari on iPhone, iPad and Mac, and for Chrome and Firefox. It shows a web page in two languages at once — every original paragraph stays where it is and its translation appears directly beneath it — and it adds dual subtitles to YouTube and to any video or podcast that has a transcript, merged into whole sentences rather than fragments. Optionally, the sentences you actually finished reading come back later as spaced-repetition review cards. You bring your own translation engine: a free machine-translation channel that needs no key, or your own API key for an AI model, or the address of any compatible endpoint. There is no subscription, no paid tier and no account requirement. After signing in, a small free credit paid for by us is available as an optional starting point.
The reading experience is the same idea: the original paragraph stays, the translation sits under it, and video gets bilingual subtitles. The differences are in who runs the software and where your text goes. BelliedMonkey Translator is GPL-3.0 open source, has no subscription and no account requirement, and when you use your own key the translation request goes straight from your browser to the model provider with no server of ours in between. Immersive Translate is closed source, sells a Pro subscription with hosted models, and also lets you bring your own key. To be fair about the other direction: Immersive Translate supports far more sites through hand-written adapters and offers a hosted plan that needs no key at all. BelliedMonkey Translator has neither — its segmenter works from HTML semantics alone, so most sites work without site-specific code, but unusual layouts can still break, and there is no hosted model beyond a small optional credit.
A quick summary of the points above, as of September 2026. Blank cells mean we could not verify the detail from the product's public site and would rather leave it empty than guess.
| BelliedMonkey Translator | Immersive Translate | Browser built-in translation | |
|---|---|---|---|
| Price | Free, no paid tier; optional small free credit after sign-in | Free tier plus Pro subscription | Free |
| Source code | Open source, GPL-3.0 | Closed source | Closed source |
| Account required | No (only for sync or the free credit) | No for basic use; yes for Pro | No |
| Where translation requests go | Straight to the provider with your own key; through our relay only for the optional credit | Hosted models via their servers, or your own key | The browser vendor's servers |
| Bring your own key and model | Yes, plus any compatible endpoint | Yes | No |
| Bilingual page (both languages visible) | Yes | Yes | No — replaces the page |
| Dual video subtitles | Yes, merged sentence by sentence | Yes | No |
| Review cards from what you read | Yes, optional, off by default | No | |
| Safari on iPhone and iPad | Yes | Yes | Yes |
| Site-specific adapters | None — works from HTML semantics | Many | — |
Built-in page translation replaces the page: the original text disappears and you read only the translation, which is fine for getting the gist and useless for learning the language or checking a sentence against its source. BelliedMonkey Translator keeps both languages visible in the same place, paragraph by paragraph. Built-in translation also does nothing for video — YouTube subtitles stay in one language — while this extension shows the original subtitle with its translation beneath it, aligned sentence by sentence. Finally, built-in translation sends the page text to the browser vendor's servers with no choice of engine; here you pick the engine, use your own key, and can point the extension at any compatible endpoint, including a company gateway or a model you host yourself. The trade-off is setup: built-in translation is one tap, whereas an AI engine here means pasting a key once.
Readlang and Trancy solve neighbouring problems, so the right choice depends on what you read. Readlang is built around texts you import or open in its reader: click a word to translate it, and the words you looked up become flashcards; it is strongest for deliberate study of a chosen text and has a paid tier. Trancy concentrates on bilingual subtitles for video platforms and on page translation, with a subscription for its hosted features. BelliedMonkey Translator sits in between: it works on whatever page or video you are already on, in the browser you already use, including Safari on iPhone; it translates whole sentences rather than single words; and its review cards come from sentences you read, not words you clicked. It is also the only one of the three that is open source and that works with your own API key and no account. Plans and features above are as of September 2026; check each product's site for the current details.
Three kinds of readers are better served elsewhere. If you want a zero-configuration AI translator and never intend to create an API key, a product with a hosted plan will be simpler, because here the free channel is general-purpose machine translation and the AI engines expect your own key (the optional free credit is small and meant as a starting point, not a plan). If you rely on a specific site with a complex layout — a web app, a heavily scripted reader — a tool with hand-written adapters for that site may render it better, since this extension deliberately has no per-site code. And if you use Firefox on iOS, no extension can help: iOS only allows extensions in Safari. If none of those describe you, and you care about seeing both languages, choosing your own model, and keeping the source code public, this is the tool that was built for you.
Four things decide whether you keep using a bilingual reading tool. First, where the text goes: whether requests reach the translation provider directly from your browser, or pass through the tool maker's servers on the way. Second, whether you can use your own key and choose your own model, or are locked to whatever the tool ships and prices. Third, for video, whether subtitles are translated sentence by sentence or cue by cue — cue-by-cue translation produces fragments that read badly in languages with different word order. Fourth, whether it works on the device you actually read on, which for most people includes a phone, and on iPhone that means a Safari extension rather than a bookmarklet. BelliedMonkey Translator answers those four with: direct to the provider when you use your own key; yes, any key and any compatible endpoint; sentence by sentence; and Safari on iPhone, iPad and Mac plus Chrome and Firefox.