Create a directory squirrelmail/locale/language_code/LC_MESSAGES/.
Copy squirrelmail/po/squirrelmail.pot into this directory and rename it
to squirrelmail.po. This is the file that is going to be translated.
Doing the actual translation:
Convert the translated squirrelmail.po into a binary file by running the
command "msgfmt -o squirrelmail.mo squirrelmail.po" in the directory
where the translated squirrelmail.po is residing.
Add "-c" option to msgfmt command if you want to be sure that your
translation is correct.
Making new translation available
Add the language name and language code to the array at the top of
Since SquirrelMail 1.5.1 new translations are enabled by creating
file named setup.php in locale/<translation_code> directory.
This file should contain $languages array entries and XTRA_CODE
functions used by translation.