diff --git a/INTERNATIONALIZATION.md b/INTERNATIONALIZATION.md
new file mode 100644
index 00000000..e26d88d7
--- /dev/null
+++ b/INTERNATIONALIZATION.md
@@ -0,0 +1,59 @@
+# Internationalization (i18n)
+
+This site uses [Lingui](https://lingui.dev/) with Next.js for internationalization. Translation catalogs are `.po` files in `src/locales/`.
+
+## Wrapping strings for translation
+
+User-facing strings are wrapped with the `` t` ` `` tagged template from `useLingui()`:
+
+```js
+import { useLingui } from '@lingui/react/macro'
+
+const MyComponent = () => {
+  const { t } = useLingui()
+  return <Text>{t`Hello world`}</Text>
+}
+```
+
+Strings with dynamic values use interpolation inside the tagged template:
+
+```js
+{
+  t`${count} items found`
+}
+```
+
+Avoid concatenating translated fragments — use a single tagged template so translators see the full sentence:
+
+```js
+// Bad: translators get disconnected fragments
+;`${value} ` + t`hours`
+
+// Good: translators see the full string
+t`${value} hours`
+```
+
+## Loading catalogs in pages
+
+Any page that uses translated strings (directly or through shared components like Header/Footer) needs to load the translation catalog in `getStaticProps`:
+
+```js
+import { loadCatalog } from '../i18n'
+
+export const getStaticProps = async (ctx) => {
+  const translation = await loadCatalog(ctx.locale)
+  return {
+    props: {
+      translation,
+    },
+  }
+}
+```
+
+Without this, translations will not load when users visit the page directly. Blog posts are an exception — they are not translated and do not need this.
+
+## Adding a new locale
+
+1. Add the locale code to `src/config/i18n.mjs`
+2. Run `npx lingui extract` to create the new `.po` file
+3. The new locale will be available for translation on Crowdin after merging
diff --git a/README.md b/README.md
index 32e8e2e9..4660138a 100644
--- a/README.md
+++ b/README.md
@@ -59,6 +59,18 @@ When creating new content or editing existing content, only the english version
 
 For more details on how the integration works, see https://scientific-python-translations.github.io/docs/. For more details on how to translate the website, see https://scientific-python-translations.github.io/translate/.
 
+### Modifying translatable strings
+
+This site uses [Lingui](https://lingui.dev/) for internationalization. When you add or modify user-facing strings, you must update the message catalogs before committing:
+
+```bash
+npx lingui extract
+```
+
+This updates the `.po` files in `src/locales/` with new message entries. You do **not** need to run `lingui compile` — the build handles this automatically. Blog posts are not translated and do not need to use Lingui.
+
+See [INTERNATIONALIZATION.md](INTERNATIONALIZATION.md) for details on how to write translatable code.
+
 ## Authoring blog post tips
 
 1. To create a new blog post a good place to start is copying a subfolder under `src/posts/`, so, for example https://xarray.dev/blog/flox is written here https://github.com/xarray-contrib/xarray.dev/blob/e04905f5ea039eb2eb848c0b4945beee323900e4/src/posts/flox/index.md