My children (6&4 yrs old) are struggling to keep all their 4 languages going. English has taken precedence as we live in London and it's the language everyone in the family has in common. Dad provides English & Urdu, I provide Japanese & Spanish. I'm working hard at keeping spanish going (my mother tongue) but my question is: when/at what age shall I crack down on their 3rd language (my second) Japanese, as I really want them both to be fluent. At the moment they have a good basic understanding of it, but that's it, they don't have any vocabulary.