As Soon as you can do so easily or with minimal effort/strain on yourself or your baby. You don't HAVE to teach a new language, your child can learn now or later. You might look into something like Little Pim or YBCR Spanish...Or you may wait until you child is older.
OR, you could start with ASL! There are tons of resources online to get you started for free and keep you going for a long time.
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If you want to start a verbal, modern language, do so. Get a simple, easy book and educate yourself and ease into the routine. Don't try to do everything at once if you're not comfortable with the next step, you can take your time and consider what you want for your baby and what is feasible for you both.
Pick a language, and just learn 1 new word a day, and use that word. IF you decide on Spanish, learn one new Spanish word a day and when you're talking to your baby, use that word.
"Just a minute, mama needs a little aqua to drink, okay?"
"Espere, baby, hold on just a little minute while mama does this"
"Oh, you picked a flower, bueno!"
Use a new word