I think the harder marking talked about in the Guardian article (based on a letter from academics in MFL) is inevitably going to put students off, especially at A level. If you know you are likely to get a lower grade then why would you risk it? When you combine that with grade inflation from native speakers, problems in recruitment and retention of teachers and often a general lack of interest in other languages from parents, then you can see why there is a problem.
From the Guardian article:
"At A-level the situation is even starker, down by more than one-third in French and 45% in German since 2010. As a result, the number going on to study languages at university in the UK has also fallen 12% since 2013, causing growing numbers of universities to cut back or even scrap language degrees.
Part of the reason for these declines is both the difficulty of languages compared with other subjects and what many experts believe is harsh marking. This means that at GCSE, languages have typically been marked half a grade more severely than other Ebacc subjects. And the introduction of new GCSEs has made the problem worse."