I had a hard time to set up my local language. Now I have done with the most of it. I like to share the process for your information.

#1: Locale language needs to be defined in the following screens in backoffice (you will see the option there if you click the following buttons):

Setting, then regional
Blog, then invidual blog
User profile
New Post
Browse/Edit

#2: Also considering editing the file /conf/_locales.php according to e2b's document howto:

a. add an entry to the $locales array by duplicating an existing entry. Customize the locale, the charset you have used for translation and finaly enter the default date and time formats for your country.

b. you'll probably want to make the newly added locale your default locale (used in the backoffice and for notification messages). Do this by setting $default_locale. You will also have to update the locale for your user profile.

c. decide if you want b2evolution to localize with the help of the gettext runtime functions ($use_l10n = 1) or with its own l10n system ($use_l10n = 2). The later is generally better. (Note: there are situations where using the gettext runtime would fail miserably...). We strongly recommend you use the default setting (2).
d. at the end of the file set $allow_po_extraction = 1;

#3, check your skin's _main.php. Find:

<?php
locale_temp_switch( $Item->locales ); // Temporarily switch to post locale
$Item->anchor(); // Anchor for permalinks to refer to
?>

Note it is "$Item->locales". If it is "$Item->locale" correct it.

After the above process you should be fine.