Several teachers at the Languages Show Live on Friday asked for advice on the number of languages pupils should study in Year 7, mostly because they were concerned that some were being given too many at once. This is not an easy question to answer, as I know of no research that addresses the issue specifically - ie we have no direct evidence.
However, there is evidence in each school, and we can ask these questions:
How many pupils take 2 languages to GCSE? The answer is, usually, very few, though this may vary between schools. If pupils are taking two languages to GCSE in significant numbers, there is a strong case for taking two at KS3, though not necessarily for starting both in Y7.
Next, how many pupils drop out of languages at 14, and why? Is spreading language learning over more than one language at an early stage a factor in this?
Third, what is the pattern of grades among those taking GCSE? Is there an appropriate pattern of grade B and above, that might prepare pupils for A level?
Finally (for the moment) what is the A level take-up, and what are the reasons for this?
It will be clear from these questions that, in my view, the answer to the question is based on evidence of outcomes in terms of pupils' attainment in languages, and it is equally clear to me that many of those in positions of authority in languages over the past fifty years have wanted attainment to take a back seat to what they term "inclusion", and I term "dropout". Starting a language to drop it at 14 is, for most pupils, a most dispiriting experience - they feel they have wasted their time, and usually, though not always, with good reason.
Going through the above questions, and considering the answers, is likely in most cases to favour one language in Year 7, with a second, or an option to change, in Y8. Schools should also offer a choice of languages to pupils starting in Year 7.