The Office for Standards in Education praised new training courses that provide a primary postgraduate certificate in education with a specialism in French as "relevant and practical".
But it said that training colleges struggled to find schools that could support such trainees.
Ofsted said many trainees were expected to introduce French while on their placement. "Trainees whose specialist teaching was limited to an extra-curricular club fared even worse, as the circumstances were too far removed from a conventional teaching context," it said.
It recommended that colleges give better support to trainees in schools which are unable to give specialist language advice.
The course includes a four-week placement in France. Ofsted found the main difficulty with this was that course managers "seriously underestimated" the time needed to arrange the visits.
But it said the visits themselves worked well and all trainees said their French improved.
David Bell, the chief inspector, said: "This group of trainees clearly enjoys teaching French in primary schools and more than 90 per cent of those who enrol on to the modern foreign languages course complete it.
Most... teach the subject well or very well."
Assessing pupils was trainees' main weakness, with some wrongly believing it was not possible to say how pupils were progressing as they were not teaching reading and writing.
The report said: "The great majority of trainees showed clear potential for subject leadership; they taught well, had good subject knowledge and were developing a good understanding of primary language issues."
The report said that trainees often used English unnecessarily and lacked strategies for giving instructions in French. The scheme began in September 2001 with 61 trainees in five colleges. The inspection covered the five institutions in their second year of runnning the scheme.
This year, the Teacher Training Agency has provided 378 initial teacher training places for primary French and a further 121 places for German and Spanish.
The Government's National Languages Strategy launched last year aims to give every primary child the opportunity to study a language by 2010.