[PeterT]

On http://calibre-ebook.com/get-involved it says

Contributing translations to calibre

calibre has support for translating its user interface. You can see the list of languages for which translations exist here. If the translations for your language are incomplete/missing, you can contribute the translations using a simple web-based interface. To do so first register/Log in at Launchpad, then click on your language in this page. You can filter the results to show only untranslated strings, and contribute translations for them.

Alternatively you can download the PO files and edit the PO file you want to update. Click on the corresponding language here and upload the PO file as "User upload". You have to be logged in to upload files.

Translations FAQ

• What to do with the & symbols?
  The & symbols denote a keyboard shortcut. So for example the string "&Title" means that the corresponding GUI element can be accessed with the keyboard shortcut "Alt+t" (this only works on windows/linux). So in your translation leave the & symbol before the same letter if possible, or some other suitable letter.
• What to do with the "%A_Word" characters?
  The % character and a word to the right of it (no space between the percentage sign and the word), denotes a word that will be replaced by the code. So when encountering this symbol and a word right next to it (example: %default), don't change it. Keep it as is in the translated sentence. Similarly for sequences like %s or %d, leave them as is. Finally, sequences like %(some_word)s should also be left unchanged.
• What do do with date strings like dd MMM yyy?
  These must be left unstranslated.
• How do I test my translations?
  First you need to download a .po file for your language from here. You can test your .po file by running calibre from the command line from the directory in which the .po file resides. This is only tested in Linux. On windows also it should work. On OSX, since you can't launch the GUI from the command line, try putting the .po file in your home directory. Note that the .po file must be named according to the language, so for german it would be de.po, for french fr.po and so on. Normally, when getting the file from Launchpad, it is named correctly automatically.