So the girls were away for 4 weeks recently, visiting the grand parents. They were immersed in a completely Arabic environment and Lilia's Arabic did obviously improve massively.
Thanks to Skype, I spoke with her almost every day and so her German did not deteriorate at all. Good stuff.
Now we're all back in England, and Lilia went to school three mornings and met her friends a couple of times, and suddenly she is back to mixing English into the German, something she had completely forgotten about.
"Ich habe salad geplanted" ("I planted salad")
The grammar is correct from a German point of you, of course.