Ah - when you add these (in the cms/lang/en_US.php and the other languages):

$lang['en_US']['PageCommentInterface.ss']['PBY'] = 'Posted by';
$lang['en_US']['PageCommentInterface.ss']['APPROVE'] = 'approve this comment';
$lang['en_US']['PageCommentInterface.ss']['ISSPAM'] = 'this comment is spam';
$lang['en_US']['PageCommentInterface.ss']['ISNTSPAM'] = 'this comment is not spam';
$lang['en_US']['PageCommentInterface.ss']['REMCOM'] = 'remove this comment';

it will work! Thank you for helping me!!! Great! I tried it before in different ways with no result but that's it! I guess the i18n translation is al little bit shaky because you have to find the included files. And sometimes there are strange effects in the results.....
Cheers,
tiga

Glad I could help. And even better to hear that it works :)

Generally, I don't get the point why it makes sense to have such a "deep" tree structure in the LOCALE stuff:

$lang['en_US']['PageCommentInterface_singlecomment.ss']['REMCOM'] = "...";

Why not just $lang['en_US']['REMCOM'] = "..."; ?

There should be at least something like
$lang['en_US']['default']['REMCOM'] = "...";

This way, one *could* still define different names for different classes *optionally*. At this moment it seems like we had to re-define the comment (and probably some more) translations for the different classes, which is a not-so-nice redundancy.

By the way, does anybody know how to place edited translation files from /cms/lang/xx_XX.php into a project-specific directory, so that silverstripe can be upgraded without overwriting them?

попробуйте так: ))
edit SSViewer.php. replace this

	public static function getTemplateContent($identifier) {
		return file_get_contents(SSViewer::getTemplateFile($identifier));
	}

	public static function getTemplateContent($identifier) {
		$content = file_get_contents(SSViewer::getTemplateFile($identifier));
		$content = ereg_replace('<' . '% +_t\((\'([^\']*)\'|"([^"]*)")(([^)]|\)[^ ]|\) +[^% ])*)\) +%' . '>', '<?= _t(\''. $identifier . '.ss.\\2\\3\'\\4) ?>', $content);
		$content = ereg_replace('<' . '% +sprintf\(_t\((\'([^\']*)\'|"([^"]*)")(([^)]|\)[^ ]|\) +[^% ])*)\),\<\?= +([^\?]*) +\?\>) +%' . '>', '<?= sprintf(_t(\''. $identifier . '.ss.\\2\\3\'\\4),\\6) ?>', $content);
		return $content;
	}