In order to create or update a translatable file, run the following command
in the directory i18n/:

./update.sh xx

where xx is the two-letter language code according to ISO 639 (which,
if needed, may be suffixed by a country code according to ISO 3166).
Quassel will automatically load these translation files based on the
system locale. Examples for valid codes: de, en_US

You can open translation files with poedit for example and add/edit translated strings.

The *.po files can and should be kept in sync with the source code by running
update.sh regularly (existing translations won't be removed).

NOTE: Remember to remove X-Virgin-Header line if you want to preserve your header.

Qt itself does not use the *.po files, but a compact binary format (*.qm). 
These files will automatically be generated at build time. By default, all
languages will be built. To select languages, add -DLINGUAS="<languages>"
to your cmake call, where <languages> is a space-separated list of language
codes as explained above.

NOTE: You'll need lconvert installed with your Qt in order to generate
      translation files. Some distributions don't package that tool;
      in that case you won't get translations.