One of the major statements the speaker makes in this long poem is that he is essentially intertwined with nature and nature (and the entire universe) is intertwined with him. In short, everything is connected.
When we think of a person or a "self," we think of an individual, a singular self differentiated from all the other selves. In "Song of Myself," the speaker celebrates him(self) but also speaks as having transcended the self. Therefore, any previous notions of an isolated self do not apply. The speaker is therefore not limited by the notion of an isolated self; he has transcended the notion. He is also therefore, not "tamed" by his "self."
Since it is uncommon to speak of one's self as having transcended the notion of self, his way of speaking is untranslatable (having never been heard or translated). This could also mean that what he is saying is unique or new and is therefore like a new language; as yet untranslatable since no one has ever heard it. Whitman/the speaker also celebrates the physical pleasures of life so this "self-transcendence" is not a transcendence over and above the physical; rather, it is a different way of looking at a self.
He compares his proclamation to a "barbaric yawp" - natural and genuine. A "yawp" to a human might sound like a dog's bark, and is therefore, untranslatable. This also means that it is open to interpretation, contradictions, and multiple meanings and impressions. Earlier in the poem (Section 51), the speaker claims:
Do I contradict myself?
Very well then I contradict myself,
(I am large, I contain multitudes.)
Multitudes: he is not limited to a singular way of experiencing. Following this, he is not limited to a singular way of expression/speaking.