Practicing by yourself is quite possible, It depends on how much you recall, but conversing with others in another language will give you mixed results within the dream. You might get better results if you were in a place where people usually spoke in those language to stabilize it a bit. This reminds me of the time when I was little I had a dream where I was in an unfamiliar place. The language people in the dream talked were also something I didn't know. In later years going over my old journals I realize it was mandarin. I wish I had note down more of these non-English dreams in detail.