source.json: every English text block found on the built pages. The key is a hash of the text, so changing an English sentence makes its old translations stop matching and that sentence shows in English until it is translated again.
pages.json: which texts appear on which page. A page counts as translated in a language when at least 80% of its texts are translated (or tools.json has an entry for it). Translated pages get their own canonical URL and hreflang links, and are listed as language alternates of the English page in the sitemap.
<lang>/content.json: hash to translation.
ui-source.json: English interface messages used by the JavaScript (trText, trHtml). Translations are in public/locales/<lang>/ui.json, keyed by the English text.

Terms, Privacy and Licensing stay English; only their navbar and footer are translated. The blog stays English.

Markup in texts

Links and formatting are replaced by numbered markers: Read <1>the docs</1> <2/>. A translation must keep every marker once. Text inside <1>...</1> is translated; <2/> is an icon or code and is copied. Interface messages use {{name}} variables instead.

Refreshing after English changes

Build the site (npm run build, and once with SIMPLE_MODE=true into another folder).
node scripts/extract-content-segments.mjs dist /path/to/simple-dist rewrites source.json and pages.json.
node scripts/extract-ui-strings.mjs rewrites ui-source.json.
node scripts/translations.mjs status and node scripts/ui-translations.mjs status show what is missing per language.
node scripts/translations.mjs chunk <workDir> --lang <lang> writes chunks with only the texts still missing in that language (without --lang it writes every text). After translating, node scripts/translations.mjs merge <lang> <workDir>/out/<lang> validates and adds the new translations; it never overwrites an existing translation unless you pass --replace, and never deletes anything. validate <lang> re-checks a language at any time. Translations of texts that no longer exist in source.json are kept and reported as stale; prune <lang> removes them once you are sure the extraction was complete (both builds).

Writing UI text in code

Text shown by JavaScript goes through trText('English text {{name}}', { name }) (plain text) or trHtml (inside innerHTML templates). showAlert, showLoader, showAlertModal and showModal translate their arguments themselves, so pass them plain string literals. For templates with values, use trText with {{name}} variables.
Do not keep translated text in module-level constants: they are evaluated before the language loads. Build them inside a function that runs when the UI is drawn.
Page setup code runs inside onPageReady(...) (from src/js/i18n), which waits for both the DOM and the translations.
Code that runs in a Web Worker cannot use trText. Pass the translated templates in from the main thread (see diffStrings in src/js/compare/worker-api.ts).
src/tests/ui-text-i18n.test.ts fails on raw English in the common UI sinks and when ui-source.json is out of date. Run node scripts/extract-ui-strings.mjs after changing UI text. node scripts/raw-ui-text-scan.mjs lists the offending lines.

Docs

Docs translations are Markdown files in docs/<lang>/, with menu labels in docs/.vitepress/i18n/<lang>.json. See scripts/docs-translations.mjs (check, localize, labels). A language is only built into the docs when every English page has a translation. Set DOCS_LANGUAGES=de,fr (or en alone) to limit which languages are built.