Having a 21mo, it's clear that her limit to absorb is greater than our ability to produce, and that's with her hearing English and Chinese. I do feel like it's a critical time, and do my best to explain things to her, even when I don't think she understands the entirety of what I am saying.
My wife get's crazy looks from family members for accurately describing how biological processes work using correct terminology to our three year old. The result is a kid that says things like, "grandpa your body makes sweat and you have sweat glands" and "this tastes good on my taste buds." It's pretty funny but also really encouraging. She has an amazing vocabulary for her age and I attribute this largely to us not talking down to her but actually taking the time to answer the seemingly unending amount of questions she has. -It's a blast.
I think I've read in the past that scientists theorize/assume that ubiquitous "baby talk" really hinders a kid's vocab/grammar/eloquence development. Makes complete sense. I wouldn't be surprised if the biggest thing the average parent could change that would make a difference in their children's development would simply be to eradicate baby talk and talking down to children completely.
Child development is a really contentious field, which makes sense as people are willing to invest in children (go figure). I've heard this too. Another thing I've read is that the way a baby is carried can affect things as well. What I read mostly discussed whether a baby being held so that its line of sight is over the back of the parent has a different effect than carrying the baby in such a way that it faces forward, can affect how independent a child might become. The article I read (if I'm remembering it right) made the case that a child is initially unable to conceive of itself as a separate entity from its parent until a given stage of development and so carrying the kid in a forward-facing manner will impart it with the idea or impression of greater control, rather than being hauled around like a sack of potatoes. I'm sure someone will come along and argue with that though.
At roughly that age, or maybe earlier, my parents wrote out labels describing every piece of furniture or object in our apartment. Everything, walls, door knobs, book titles and so on. You know how some people are really good at puzzles and some people just spatially are clueless as far as isolating matching pieces? I'm convinced that in large part comes from early childhood exposure to puzzles. My parents always left puzzles lying around that had maps on them, or facts, or some sort of writing in general. Same with books, brain games, etc. Obviously some of that applies to age 1.5 and some to ages 5-10, but the idea is -- parent even when not physically present. Let your daughter learn on her own when you and your wife are working. Might foster independence, love of knowledge, could solve the problem of you not being able to teach at all the times she's receptive of information. At the very least she'll have an understanding not commensurate with her age of things like draperies and floorboards. Paint swatches. Bed frames.