Is it possible to use set the locale manually in a template?
Completely, 100% untested, but I think you'd be able to pull it off with a pretty simple plugin that added a new Twig filter (check the Twig Extensions switch).

From a template you'd something like:

{{ myDate|myDateFilter('es') }}


Then your plugin's myDateFilter method would do something like:

public function myDateFilter(\Twig_Environment $env, $date, $locale)
{
    $localeData = craft()-&gt;i18n-&gt;getLocaleData($locale);
    $dateFormatter = $localeData-&gt;getDateFormatter();

    return $dateFormatter-&gt;formatDateTime($date-&gt;getTimestamp());
}