As Eli proposed I'm forwarding this mail thread to thedevel list.
Short summary: po-mode.el coming with GNU gettext helps to edit so
called PO files.  Thanslators provide the 'msgstr ""' fields;
'msgid ""' fields containing the original program messages:

#: plugins/dif/dif.c:77 plugins/excel/ms-excel-read.c:4076
msgid "Reading file..."
msgstr "Leyendo archivo..."

The tough part is the very first message of every file holding meta
info:

msgid ""
msgstr ""
"Project-Id-Version: gnumeric 0.69\n"
"POT-Creation-Date: 2001-07-17 22:49-0400\n"
"PO-Revision-Date: 2001-08-12 16:34GMT\n"
"Last-Translator: Germán Poo Caamaño <gpoo@ubiobio.cl>\n"
"Language-Team: es <es@li.org>\n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=iso-8859-1\n"
"Content-Transfer-Encoding: 8bit\n"

Emacs has to learn to look for the charset info and choose the correct
coding.

Here are the old mails: