Translation docs typo fix and translation update

Shall I delete the unused lines in the Chinese translation?
```PHP
// English
$lang['exp_forever'] = 'Keep Forever';
$lang['exp_burn'] = 'Burn on reading';
$lang['exp_5min'] = '5 Minutes';
$lang['exp_1h'] = '1 Hour';
$lang['exp_1d'] = '1 Day';
$lang['exp_1w'] = '1 Week';
$lang['exp_1m'] = '1 Month';
$lang['exp_1y'] = '1 Year';
// Chinese
$lang['exp_forever'] = '永久保留';
$lang['exp_burn'] = '閱後即焚';
$lang['exp_5min'] = '五分鐘';
$lang['exp_30min'] = '半小時';	// unused
$lang['exp_1h'] = '一小時';
$lang['exp_6h'] = '六小時';	    // unused
$lang['exp_12h'] = '半天';	   // unused
$lang['exp_1d'] = '一天';
$lang['exp_1w'] = '一週';
$lang['exp_4w'] = '四週';	       // unused
$lang['exp_1m'] = '一月';
$lang['exp_1y'] = '一年';
```