how to a translatable string in drupal theme?
The first step to translate a string used from a module, or a theme is using t() for every string that needs to be translated. Remember that the first argument given to the function is a literal string containing a phrase in English. This means that the following calls to t() are not correct.

// The string is a concatenation of literal strings.
$error = t('This example, ' . "doesn't work.");




// Also in this case there is a concatenation of literal strings.
$errno = 15;
$error = t("Error number $errno received while translating a string.");




// This could work, provided that another function calls t() using a
// literal string that is equal to the string returned from the function.
$error = t(mymodule_get_error());


The second step is providing the translation of the string passed to t(). If the theme is hosted on Drupal.org, then the translation will be provided from translators on http://localize.drupal.org.
Otherwise, you need to translate the string on admin/build/translate/search.

You can look for the string to translate searching for untranslated strings, or entering a string.