That bug has been described on https://launchpad.net/distros/ubuntu/+source/evolution/+bug/44081

"

The strings "Yesterday" and "Today" appears in the Date field when you browse your messages.

These strings are translated into Spanish to "Ayer" and "Hoy", respectively, and these translations are in the .mo file, as you can see:

ricardo@yuggoth:~ $ strings -eS /usr/share/locale-langpack/es/LC_MESSAGES/evolution-2.6.mo | grep "Ayer"
Ayer %l:%M %p
ricardo@yuggoth:~ $ strings -eS /usr/share/locale-langpack/es/LC_MESSAGES/evolution-2.6.mo | grep "Hoy"
Hoy a las %H:%M
Hoy a las %H:%M %S
Hoy %l:%M %p
Hoy a las %l:%M:%S %p
ricardo@yuggoth:~ $

However, the Spanish strings doesn't appears, as you can see in the screenshot I post.

http://librarian.launchpad.net/2558293/evolution.png
"Yesterday" must be "Ayer", and "Today" must be "Hoy". The translations are in the .mo file, but are not applied in the application."