Hellou! I make this:

mkdir /locales/es_ES (Correspondiente to "España - castellano")

mkdir /locales/es_ES/LC_MESSAGES

And use the messages.po (If this date, of 618 lines only 337 fault to traslate)

Two questions:

I edit the file /conf/_locales.php (using the last beta 8.5.3):

<?php
/*
* b2evolution localization & language config
* Version of this file: 0.8.3
*
* Reminder: everything that starts with #, /* or // is a comment
*/
# Enable localization?
# set to 0 to disable localization
# set to 1 to enable gettext localization if supported (not recommended)
# set to 2 to enable b2evo advanced localization (recommended)
$use_l10n = 2;

# To be used for m17n support:
$dbcharset = 'iso-8859-1'; // If you don't know, don't change this setting.

// Load locale related functions: (ne need NT_() here)
require_once (dirname(__FILE__)."/$conf_dirout/$core_subdir/_functions_locale.php");

# Supported languages for posts:
$languages = array(
'en' => NT_('English'),
'fr' => NT_('French'),
'ja' => NT_('Japanese'),
'nl' => NT_('Dutch'),
'pt' => NT_('Portuguese'),
'sv' => NT_('Swedish'),
'es' => NT_('Spanish'),
);

# Agregada cadena es y puesto todas las anteriores como comentario
# Default locale used for backoffice and notification messages
# These use an ISO 639 language code, a '_' and an ISO 3166 country code
# This MUST BE in the list below
$default_locale = 'es_ES';

# and localization:
# Add what you need and comment what you don't need
$locales = array(
'en_US' => array( // English, US
'charset' => 'iso-8859-1', // gettext will convert to this
'datefmt' => 'm/d/y',
'timefmt' => 'h:i:s a',
),
'fr_FR' => array( // French
'charset' => 'iso-8859-1',
'datefmt' => 'd.m.y',
'timefmt' => 'H:i:s',
),
'nl_NL' => array( // Dutch
'charset' => 'iso-8859-1',
'datefmt' => 'd-m-y',
'timefmt' => 'H:i:s',
),
'ja_JP' => array( // Japanese
'charset' => 'utf-8',
'datefmt' => 'Y/m/d',
'timefmt' => 'H:i:s',
),
'pt_BR' => array( // Portuguese, BRAZIL
'charset' => 'iso-8859-1',
'datefmt' => 'd/m/y',
'timefmt' => 'H:i:s',
),
'sv_SE' => array( // Sweedish, SWEDEN
'charset' => 'iso-8859-1',
'datefmt' => 'y-m-d',
'timefmt' => 'H:i:s',
),
'es_ES' => array( // Spain, Castellano
'charset' => 'iso-8859-1',
'datefmt' => 'd-F-Y',
'timefmt' => 'H:i',
),
);

/* How to format the dates and times:
The following characters are recognized in the format string:
a - "am" or "pm"
A - "AM" or "PM"
B - Swatch Internet time
d - day of the month, 2 digits with leading zeros; i.e. "01" to "31"
D - day of the week, textual, 3 letters; i.e. "Fri"
F - month, textual, long; i.e. "January"
g - hour, 12-hour format without leading zeros; i.e. "1" to "12"
G - hour, 24-hour format without leading zeros; i.e. "0" to "23"
h - hour, 12-hour format; i.e. "01" to "12"
H - hour, 24-hour format; i.e. "00" to "23"
i - minutes; i.e. "00" to "59"
I (capital i) - "1" if Daylight Savings Time, "0" otherwise.
j - day of the month without leading zeros; i.e. "1" to "31"
l (lowercase 'L') - day of the week, textual, long; i.e. "Friday"
L - boolean for whether it is a leap year; i.e. "0" or "1"
m - month; i.e. "01" to "12"
M - month, textual, 3 letters; i.e. "Jan"
n - month without leading zeros; i.e. "1" to "12"
r - RFC 822 formatted date; i.e. "Thu, 21 Dec 2000 16:01:07 +0200" (added in PHP 4.0.4)
s - seconds; i.e. "00" to "59"
S - English ordinal suffix, textual, 2 characters; i.e. "th", "nd"
t - number of days in the given month; i.e. "28" to "31"
T - Timezone setting of this machine; i.e. "MDT"
U - seconds since the epoch
w - day of the week, numeric, i.e. "0" (Sunday) to "6" (Saturday)
Y - year, 4 digits; i.e. "1999"
y - year, 2 digits; i.e. "99"
z - day of the year; i.e. "0" to "365"
Z - timezone offset in seconds (i.e. "-43200" to "43200"). The offset for timezones west of UTC is always negative, and for those east of UTC is always positive.

Unrecognized characters in the format string will be printed as-is.
*/

# Default language (ISO code)
# We get this one from the default locale above
$default_language = substr( $default_locale, 0, 2 );

# day at the start of the week: 0 for Sunday, 1 for Monday, 2 for Tuesday, etc
$start_of_week = 1;

# Set this to 1 if you are a translator and wish to use the .po extraction script in
# the /locales folder. Do not allow this on production servers as people could harm
# your operations by continuously recomputing your language files.
$allow_po_extraction = 1;
?>

¿If okay?

Second question:

On finish the work send the *.po and *.mo to ¿¿?? (I preferrer email :P)
and... if this work is good (wait of yes xD) ¿want time I am wait to view included on the last release? :P