I'm an MB of a 9 mo. I have gradually gotten less chatty over time because it can be exhausting. But I know that talking a lot is really valuable so I try. I find that it's easiest to talk through what I'm doing with them rather than be randomly chatty during floor play. So I talk to my 9 mo. a lot during diaper changes describing what I'm doing (and asking her to "help", though I usually help her "help"). We chat about the food she eats at meal times. I tell her how much I love her during nursing sessions because it seems appropriate during the cuddle times.
But yes, I find the chatting during floor play thing tough. You can only describe "oh, good job, you put the red stacking ring on the peg" so many times it seems like. When DD babbles at me I do respond and interpret, "Oh, yes, you said Dada. Dada's over there..." But sometimes I do just observe and let her play. I'm OK with our nanny doing that, too.
The PP's suggestion to talk about random things is a good one if you are concerned about talking. My H does that a lot. However from what I've read it is best to focus on talking about whatever the baby is focused on. That's why saying things like "yes, I see you there chewing on the hard, smooth, red, wooden block" at least some of the time is pretty valuable.