The Tonal and the Nagual
There is no word in your language that exactly fits as a translation for Tonalli. "Fate" is sometimes used because one's Tonal does determine, to a great extent, the course of one's life. "Soul" is used sometimes too, since the Tonal represents the Inner Fire that animates and drives a person, but the Tonal is not the self, nor is it in any sense a part of the Self left existing and conscious after bodily death.
But there is some truth to both. The Tonal, which is acquired at birth but may be lost - temporarily or permanently - is the essence of the interaction between the individual and the natural universe.
Xiuhtecuhtli, Lord of the Fire