Translating Booktype interface
There are two components of Booktype to translate: lib/booki and lib/booktypecontrol.
Booktype is a Django application and all rules for translating other Django applications should apply as well. For more info please check this document https://docs.djangoproject.com/en/1.4/topics/i18n/translation/.
Checkout the Booktype repository
If you have Github account you can just fork Booktype repository. You should checkout devel branch.
Create new locale directories (e.g. locale/de/LC_MESSAGES/ for German) for each component.
Create message file
Then in each component directory run the makemessages command (You might have to install a few gettext packages first, before the above command will work), eg for German:
You should end up with two untranslated .po files:
Update the header information in the commented part at the top of each file.
Load these .po files into Poedit and start translating. Enter the team contact information and language into your poedit Edit -> Preferences and Catalog -> Settings, they will be added to the file.
When you save in Poedit, django.mo files will be compiled.
Source text is original text you will have to transate. Notes for translators should tell you more about the context of the original text. Be aware, fuzzy translations will not be visible in the Booktype interface. You will have to remove fuzzy flag if you want them to be visible.
Now you can send .po and .mo files to the Booktype team. If you have Github account you should send pull request to the Booktype team.