At any rate, I can report that Clara's first words that I'm positive were words -- used and understood, with clear meaning, etc. -- were an emphatic "AAAH DAAAAA" (all done) at the end of dinner the other night. Clara also did the appropriate gestures for "All Done" (a sort of clapping that K must have picked up from daycare babysign and then a wild waving that Clara has added to make it sure we know she's done). The "Ahh Daa" was spoken first, clearly -- the gestures were only really added when I paused to marvel at the clarity of the words rather than getting her out of her high chair -- ah, the frustration!
This is assuming that the first word must be spoken. If not, then I have a clear first word from this past weekend (earlier still): "clap." I asked Clara to clap, for once not clapping myself or gesturing but merely saying the word, and she immediately began clapping. I tried out some other words and quickly found that she seemed to also know "wave" and "dance."
I realize that it can be hard to know for sure whether she knows these words since, understandably, her patience with being told what to do and doing it does wear thin rather quickly.
Other contenders for first-word (i.e. other things we think we've heard her say) include: "Clara" and "Hello."
How I know what counts as first, I know not, but I can definitely say that Clara is using language now -- let the fun begin!