Add docs on how to i18n

internal/i18n/README.md

@@ -0,0 +1,31 @@
+# Internationalization
+
+## New language
+
+To localize into a new language:
+
+1.  Create a folder in `locales/` with the ISO 639-1 (preferred) or ISO 639-2
+    code.
+
+1.  Copy the canonical example from `locales/en/default.po`:
+
+    ```text
+    cp locales/en/default.po locales/<lang>/default.po
+    ```
+
+1.  Translate the `msgstr` values. Do **not** change other values in the file.
+
+1.  Save the file and submit a pull request.
+
+
+## Update an existing language
+
+To update an existing language:
+
+1.  Find the language in the `locales/` folder.
+
+1.  Update the `msgstr` fields to the new values.
+
+1.  Save the file.
+
+1.  Submit a pull request.