The other day I overheard my family while the kids were in the bath and Nathan was cleaning them.
Linnea was going on and on about wanting to brush her teeth, but in Swedish: "Nea borsta tänderna, Nea borsta tänderna!" (=Linnea wants to brush her teeth etc).
Nathan didn't seem to pay her any attention (don't know if it was because he didn't understand or because he just wasn't listening...) so after a while Lucas looked at Nathan and said loudly and with irritation in his voice:
-"Daddy!! Linnea wants to brush her teeeeeth!!"
That was the first time I realized that Lucas was actually translating for her!
This means, he knows when they speak which language, and he knew that Nathan didn't understand what she said, because it was in Swedish!