What is a first language?

One of the questions a student or his/her parents is asked when enrolling at the school is, what is your first language? Now I have friends that the husband speaks German at home and the mother speaks English, now what is their first language? The statistics can be flawed because the parents think that there child will be not accepted in the school if English is not there first language, which is rubbish. I have just completed the stats for the PAT tests which look at the students reading comprehension and vocabulary, it has been a long road since the second week of school when the tests were carried out in the hall and the class teachers entered the results. So far I have had to compare 2006 with 2007, year 9 last year and year 10 this year, have they improved, gotten worse?