In plain words
One sees it as a wonder; another speaks of it as a wonder; another hears of it as a wonder. Yet even after hearing it, no one truly knows it.
Word for word
Someone sees this [one] as a wonder, and likewise another speaks [of it] as a wonder, and another hears of this [one] as a wonder; and even having heard, no one indeed knows this [one].
The commentators
Bhagavadgītābhāṣyam
दुर्विज्ञेयोऽयं प्रकृत आत्मा ; किं त्वामेवैकमुपालभे साधारणे भ्रान्तिनिमित्ते । कथं दुर्विज्ञेयोऽयमात्मा इत्यत आह —
आश्चर्यवत् आश्चर्यम् अदृष्टपूर्वम् अद्भुतम् अकस्माद्दृश्यमानं तेन तुल्यं आश्चर्यवत् आश्चर्यमिति एनम् आत्मानं पश्यति कश्चित् । आश्चर्यवत् एनं वदति तथैव च अन्यः । आश्चर्यवच्च एनमन्यः शृणोति । श्रुत्वा दृष्ट्वा उक्त्वापि एनमात्मानं वेद न चैव कश्चित् । अथवा योऽयमात्मानं पश्यति स आश्चर्यतुल्यः, यो वदति यश्च शृणोति सः अनेकसहस्रेषु कश्चिदेव भवति । अतो दुर्बोध आत्मा इत्यभिप्रायः ॥ २९ ॥
In plain words
This Self we have been speaking of is genuinely hard to know. So why single you out for blame? The cause of the confusion is common to everyone. "But how is this Self hard to know?" — and to that the Lord says—
'Like a wonder' -- a wonder is something never seen before, astonishing, that shows up out of nowhere. That's exactly what the Self is like: for some, seeing it is a wonder; for another, speaking of it is a wonder; for another, even hearing about it is a wonder. And yet -- someone can hear about it, see it described, talk about it endlessly, and still not truly know it. Or take it another way: the person who has actually seen the Self is himself a kind of wonder. Someone who can genuinely speak of it, or genuinely hear it, is one in many thousands. That's how rare real understanding of the Self is.
Close to the Sanskrit
Difficult to know is this Self under discussion; why do I reproach you alone, the cause of the delusion being common? 'How is this Self difficult to know?' — with this in view he says —
'Like a wonder' [means]: a wonder [is] something unseen before, marvelous, appearing suddenly; equal to that is 'like a wonder' -- [in this sense] some person sees this Self as 'a wonder.' Another likewise speaks of this [Self] as 'a wonder.' And another hears of this [Self] as 'a wonder.' But even having heard, seen, or spoken [of it], no one truly knows this Self. Or else: the one who sees this Self is like a wonder [himself]; the one who speaks [of it] and the one who hears [of it] is, among many thousands, only a rare one. Hence the Self is hard to understand -- such is the purport.