Add support for translations

This implements a translation pipeline using the industry-standard
Gettext[1] system.

I picked Gettext for the reasons described in [2] and [3]:

* It’s widely used in open source software. This means that there are
  graphical editors which will help you in editing the `.po` files. An
  example is Poedit[4], which is available for all major platforms.

  There are also many online systems for doing translations. An
  example is Pontoon[5], which is used for the Rust website itself. We
  can consider setting up such an instance ourselves.

* It is a light-weight yet structured format. This means that nothing
  changes with regards to how you update the original English text. We
  can still accept fixes and PRs like normal.

  The structure means that translators can see exactly which part of
  the course they need to update after a change. This is completely
  lost if you simply copy over the original text and translate it
  in-place in the Markdown files.

The code here only adds support for translations. They are not yet
tested, published or used for anything. Next steps will be:

* Add support for switching languages via a bit of JavaScript on each
  page.

* Update the speaker notes feature to support translations (right now
  “Speaker Notes” is hard-coded into the generated HTML). I think we
  should turn it into a mdbook preprocessor instead.

* Add testing: We should test that the `.po` files are well-formed. We
  should also run `mdbook test` on each language since the
  translations can alter the embedded code.

Fixes google#115.

[1]: https://www.gnu.org/software/gettext/manual/html_node/index.html
[2]: rust-lang/mdBook#1864
[3]:
rust-lang/mdBook#5 (comment)
[4]: https://poedit.net/
[5]: https://pontoon.rust-lang.org/

.gitignore

@@ -1,2 +1,3 @@
 /book/
-/target/
+target/
+po/messages.pot

TRANSLATIONS.md

@@ -0,0 +1,116 @@
+# Translations of Comprehensive Rust 🦀
+
+We would love to have your help with translating the course into other
+languages! We use the [Gettext] system for translations. This means that you
+don't modify the Markdown files directly: instead you modify `.po` files in a
+`po/` directory. The `.po` files are small text-based translation databases.
+
+There is a `.po` file for each language. They are named after the [ISO 639]
+language codes: Danish would go into `po/da.po`, Korean would go into
+`po/ko.po`, etc. The `.po` files contain all the English text plus the
+translations. They are initialized from a `messages.pot` file (a PO template)
+which contains only the English text.
+
+We will show how to update and manipulate the `.po` and `.pot` files using the
+GNU Gettext utilities below.
+
+[Gettext]: https://www.gnu.org/software/gettext/manual/html_node/index.html
+[ISO 639]: https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
+
+## I18n Helpers
+
+We use two helpers for the translations:
+
+* `mdbook-xgettext`: This program extracts the English text. It is an mdbook
+  renderer.
+* `mdbook-gettext`: This program translates the book into a target language. It
+  is an mdbook preprocessor.
+
+Install both helpers with the following command from the root of the course:
+
+```shell
+$ cargo install --path i18n-helpers
+```
+
+## Creating and Updating Translations
+
+First, you need to know how to update the `.pot` and `.po` files.
+
+As a general rule, you should never touch the auto-generated `po/messages.pot`
+file. You should also not edit the `msgid` entries in a `po/xx.po` file. If you
+find mistakes, you need to update the original English text instead. The fixes
+to the English text will flow into the `.po` files the next time the translators
+update them.
+
+### Generating the PO Template
+
+To extract the original English text and generate a `messages.pot` file, you run
+`mdbook` with a special renderer:
+
+```shell
+$ MDBOOK_OUTPUT='{"xgettext": {"pot-file": "messages.pot"}}' \
+  mdbook build -d po
+```
+
+You will find the generated POT file as `po/messages.pot`.
+
+### Initialize a New Translation
+
+To start a new translation, first generate the `po/messages.pot` file. Then use
+`msginit` to create a `xx.po` file for the fictional `xx` language:
+
+```shell
+$ msginit -i po/messages.pot -l xx -o po/xx.po
+```
+
+You can also simply copy `po/messages.pot` to `po/xx.po`. Then update the file
+header (the first entry with `msgid ""`) to the correct language.
+
+### Updating an Existing Translation
+
+As the English text changes, translations gradually become outdated. To update
+the `po/xx.po` file with new messages, first extract the English text into a
+`po/messages.pot` template file. Then run
+
+```shell
+$ msgmerge --update po/xx.po po/messages.pot
+```
+
+Unchanged messages will stay intact, deleted messages are marked as old, and
+updated messages are marked "fuzzy". A fuzzy entry will reuse the previous
+translation: you should then go over it and update it as necessary before you
+remove the fuzzy marker.
+
+## Using Translations
+
+This will show you how to use the translations to generate localized HTML
+output.
+
+## Building a Translation
+
+To use the `po/xx.po` file for your output, run the following command:
+
+```shell
+$ MDBOOK_BOOK__LANGUAGE='xx' \
+  MDBOOK_PREPROCESSOR__GETTEXT__PO_FILE='po/xx.po' \
+  MDBOOK_PREPROCESSOR__GETTEXT__RENDERERS='["html"]' \
+  MDBOOK_PREPROCESSOR__GETTEXT__BEFORE='["svgbob"]' \
+  mdbook build -d book/xx
+```
+
+This will update the book's language to `xx`, it will make the `mdbook-gettext`
+preprocessor become active and tell it to use the `po/xx.po` file, and finally
+it will redirect the output to `book/xx`.
+
+## Serving a Translation
+
+Like normal, you can use `mdbook serve` to view your translation as you work on
+it. You use the same command as with `mdbook build` above, but additionally
+we'll tell `mdbook` to watch the `po/` directory for changes:
+
+```shell
+$ MDBOOK_BOOK__LANGUAGE=xx \
+  MDBOOK_PREPROCESSOR__GETTEXT__PO_FILE=po/xx.po
+  MDBOOK_BUILD__EXTRA_WATCH_DIRS='["po"]'
+  mdbook serve -d book/xx
+```

book.toml

@@ -8,7 +8,14 @@ title = "Comprehensive Rust 🦀"
 [rust]
 edition = "2021"
 
+[preprocessor.links]
+renderers = ["html"]
+
+[preprocessor.index]
+renderers = ["html"]
+
 [preprocessor.svgbob]
+renderers = ["html"]
 class = "bob"
 
 [output.html]