When milk is cultured (eg kefir), what changes occur in terms of lactose, glucose and galactose levels?

In general, the process of fermentation is a conversion of sugars to alcohol and carbon dioxide. Lactose being a disaccharide composed of glucose and galactose is somewhat harder for the culture to digest, so as fermentation progresses, the levels of all three sugars will fall but lactose will fall at a slower rate than galactose or glucose.