Ministers have been told by the Qualifications and Curriculum Authority that resources must be made available before languages can become part of the statutory curriculum.
A QCA-commissioned survey by Warwick University, published this week, suggests that the number of primary schools offering languages could have slightly declined over the past few years to about a fifth because of lack of time and national test pressures.
It revealed wide regional variations and said school programmes were often dependent on a single member of staff or a visiting language teacher.
Many of the 2,000 primary heads questioned felt that current provision would only expand if languages became statutory.
However, there were major concerns about curriculum overload. A number of schools which taught languages did not support making the subject compulsory because they wanted flexibility in their approach.
Some linguists in secondary schools thought that teaching the subject at primary level could do more harm than good. There was also concern that the extension of languages at key stage 2 would increase the dominance of French.
Ministers asked the QCA to produce the feasibility study after the Nuffield Inquiry claimed language learning in Britain was in a dire state and recommended that all pupils aged seven and over should learn a language.
But the QCA report said: "While statutory provision of languages is attractive to some, it could not be achieved in the short-term without considerable investment in a national programme of teacher training."
It said that if specialist teacher-training places were increased at primary level, and in-service training was funded by the Government, the supply of primary-level language teachers could rise substantially over the next five years.
In May, the Government set up a steering group to look at practical measures to develop language learning. Specialist schools are at the centre of the agenda. As part of specialist status, schools must spearhead local language learning and spread expertise to feeder primary schools.
Steven Fawkes, president of the Association of Language Learning, said the large-scale national training involved in the literacy and numeracy strategies could be mirrored in languages if there was strong political will.