The second line of Atisha’s refuge prayer completes going for refuge. So whether it is grammatically correct or not, the first two lines form the thought of going for refuge (but actually I remember my Sakya lama teaching that the first two lines cover refuge and the last two lines cover Bodhicitta).
བྱང་ཆུབ་ : chang chub : Bodhi, Enlightenment
བར་དུ་བ : bar du : until
དག : dag : I, self
ནི : ni : a particle that set’s off the subject or topic
སྐྱབས་སུ་མཆི : kyab su chi : go for refuge
Combining the two lines and putting the verb phrase in the second position (standard for English simple sentences) we get:
I go for refuge to the Buddha, Dharma and sangha, until I am Enlightened.
The phrase, “until I am Enlightened” could also come first:
Until I am Enlightened, I go for refuge to the Buddha, Dharma and Sangha
or even in the middle:
I go for refuge, until I am Enlightened, to the Buddha, Dharma and Sangha
Other translations are also possible depending on how literal one wants to be (e.g. Arya Sangha, Supreme Assembly). The translations above leave out རྣམས་ལ – nam la, to all.
Adding that we can derive:
I go for refuge, until I am Enlightened, to all the Buddhas, Dharma and Sangha.
So we have to at least pluralize Buddha (we could also pluralize Dharma and Sangha).