t() function in template.php

I've run into the same issue and have found no success as of yet. It seems this might be a more base issue with the way locale.module processes the t() function. Any one else have any ideas how to get around this? I'm still looking around for a solution to get the Manage Strings functionality to recognize strings in *.tpl.php and template.php files (and even in page or block content, for that matter).

If you don't need to pass additional arguments, you can use locale function instead :
locale('my string').

Otherwise you can add this modified function in your template.php :

//Function to translate strings in theme files
function tr($string, $args = 0, $return = FALSE) {
  global $locale;
    if (function_exists('locale') && $locale != 'en') {
      $string = locale($string);
  }
  if (!$args) {
    if($return) return $string;
    print $string;
  }
  else {
   // Transform arguments before inserting them
   foreach ($args as $key => $value) {
    switch ($key[0]) {
      // Escaped only
      case '@':
        $args[$key] = check_plain($value);
        break;
      // Escaped and placeholder
      case '%':
      default:
        $args[$key] = theme('placeholder', $value);
        break;
      // Pass-through
      case '!':
    }
  }
  $string = strtr($string, $args);
  if($return) return $string;
  print $string;
}

And use it in this way :
tr('my string');

Yeah, you can use t() function all around.

Whatever this is plain localization, nothing to do with i18n.

My drupal version is 6.20
I test it can't working.
I changed:

function tr($string, $args = 0, $return = FALSE) {
  $locale = language_default();
  if (function_exists('locale') && $locale->language != 'en') {
    $string = locale($string, $locale->language);
  }
  if (!$args) {
    return $string;
  } else {
    // Transform arguments before inserting them
    foreach ($args as $key => $value) {
      switch ($key[0]) {
        // Escaped only
        case '@':
          $args[$key] = check_plain($value);
          break;
        // Escaped and placeholder
        case '%':
        default:
          $args[$key] = theme('placeholder', $value);
          break;
        // Pass-through
        case '!':
      }
    }
    $string = strtr($string, $args);
    return $string;
  }
}

My drupal version is 6.20
I test it can't working.
I changed:

function tr($string, $args = 0, $return = FALSE) {
  $locale = language_default();
  if (function_exists('locale') && $locale->language != 'en') {
    $string = locale($string, $locale->language);
  }
  if (!$args) {
    return $string;
  } else {
    // Transform arguments before inserting them
    foreach ($args as $key => $value) {
      switch ($key[0]) {
        // Escaped only
        case '@':
          $args[$key] = check_plain($value);
          break;
        // Escaped and placeholder
        case '%':
        default:
          $args[$key] = theme('placeholder', $value);
          break;
        // Pass-through
        case '!':
      }
    }
    $string = strtr($string, $args);
    return $string;
  }
}