Education researcher and trainer Dylan Wiliam tweeted this abstract today. The study concerned has analysed 71 peer-reviewed studies in order to find out the optimal conditions for learning a second language. Here is the relevant part for teachers of French in the UK, or teachers of English in France:
(1)... L2 learners with little L2 exposure require explicit instruction to master grammar;
(2) L2 learners with strong L2 aptitude, motivation, and first language (L1) skills are more successful; (3) Effective L2 teachers demonstrate sufficient L2 proficiency, strong instructional skills, and proficiency in their students’ L1;
(4) L2 learners require 3-7 years to reach L2 proficiency, with younger learners typically taking longer but more likely to achieve close-to-native results.
I am reminded of Sybil Fawlty's specialist subject on Mastermind ("the bleedin' obvious"), but to be fair it is reassuring to know that research supports what sensible teachers know. Point (1) is notable since it refers to the need for explicit grammar teaching. There may be a minority of language teachers who would not accept this view.
Point (3) raises an interesting question: are native French speakers teaching in the UK, and perhaps with imperfect English, at a disadvantage in some cases. Despite the research findings it would be unwise to generalise, since so many factors come into play when you try to measure successful teaching.
As regards point (4) I wonder what is meant by proficiency, but my impression would be that when a L2 learner is learning in a L2 environment, then 3-7 years may be reasonable. However, when the learner of French is in the UK, then 3-7 years appears optimistic for most students.
To my mind, natural ability is the major factor, followed by motivation. There is a curious view around at the moment that there may be no such thing as ability and that all students can achieve high levels with enough time and practice. The argument goes that David Beckham became brilliant at free kicks becase he practised them hundreds of times, not because he was naturally good at kicking a ball. Practice makes perfect and ability is not the key.
It may be true that all young children can acquire languages, but once students are older all teachers know that the ability to master new languages varies enormously and that some children just can't do it, or at least they would need lots of time and immersion to achieve proficiency.