DW is bilingual (Spanish/English), I am not. We are committed to raising DD bilingual, but not equally. English mastery takes priority, and conversational Spanish is good enough (though DD will be free to take it as far as she likes). DW began teaching DD Spanish once it was decided DD had a comfortable-enough grasp of English. She was around 3-4yo before they started.
All of DD's reading instruction so far has been in English, though if you know the basic phonics and are familiar with the language, reading in Spanish is shockingly easy by comparison. Like Japanese, it's entirely phonetic. I once read my DW her emails from Spanish family members when she couldn't come to the computer, and I was getting the pronunciations right, yet had no idea what I was saying. She thought that was hilarious, though reading it like an over-caffeinated soccer announcer may have played a part.
At certain parts of the day at home, DD and DW converse entirely in Spanish. They also visit family (here and in Mexico) and speak Spanish there. DD has even declined to switch to the English-language feeds of her favorite television shows while she's visiting in Mexico.
