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.

Tip: You should not edit the .po files by hand. Instead use a PO editor, such as Poedit. There are also several online editors available. This will ensure that the file is encoded correctly.

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.

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:

$ cargo install --path i18n-helpers

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.

To extract the original English text and generate a messages.pot file, you run mdbook with a special renderer:

$ MDBOOK_OUTPUT='{"xgettext": {"pot-file": "messages.pot"}}' \
  mdbook build -d po

You will find the generated POT file as po/messages.pot.

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:

$ 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.

As the English text changes, translations gradually become outdated.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

$ 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.

This will show you how to use the translations to generate localized HTML output.

Note: mdbook will use original untranslated entries for all entries marked as "fuzzy" (visible as "Needs work" in Poedit). This is especially important when using cloud-translate for initial translation as all entries will be marked as "fuzzy".

To use the po/xx.po file for your output, run the following command:

$ MDBOOK_BOOK__LANGUAGE=xx 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.

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:

$ MDBOOK_BOOK__LANGUAGE=xx mdbook serve -d book/xx