I guess the school is a language college specialist school? I know a couple of these schools but neither operate a linguistic skills test.
I would imagine that it could be very much like verbal reasoning. Testing how dextrous children are with using their own language, etc. They may also have lists of words and ask the children to decide whether they are French, German, Spanish or Italian, in other words, how do the words look and sound to the children and do they have a natural ability (based on their miniscule if any experience) to hear and see language differences. However, I am not a teacher so I am not sure on this one.
I recommend that you go and ask on the TES (Times Educational Suppliment) web site. The address is www.tes.co.uk
and in the left column you will see a button for staffroom. This is a forum rather like this and one of the sections under the "Staffroom" is MFL (Modern Foreign Languages). You simply follow the instructions to register on the web site and you can then post your question on the MFL postings section. There always seems to be a large number of posts on a daily basis in each section (I read it because I find the whole thing quite fascinating!!) and you may well get your information there.
Good luck and I'll look out for your post!