Professor Colin Williams of Cardiff University told the annual conference of Comunn na Gaidhlig in Nairn last week that Wales was "having to eat humble pie" after discovering that bilingual students were no better at learning French, Spanish, Russian and German.
"We have to be relatively guarded in the way in which we promote bilingualism," Professor Williams said. "Products of bilingual schools are less likely to be apprehensive and are more willing to make mistakes and have mental gymnastics going on in their head when they are translating from one language to another, but it does not automatically follow that you can be fully functionally trilingual."
He believed the assertion that it became easier to learn other European languages was because those who first went through bilingual education had strong support and were made to work hard. As the number of bilingual schools developed, that advantage diminished.
Professor Williams, a key government adviser in Wales, said that for the past two years Welsh language had been compulsory from the age of five to 16 with three lessons a week in secondary. On top of that 26 per cent of primary children were in bilingual schools and 22 per cent of secondary pupils. Around 19 per cent of a population of 3 million spoke Welsh.
He believed, however, that compulsory Welsh "presented a major challenge to the mindset of the perception of the majority of Welsh parents" - a fifth of the population were not born in the country and movement back and forth from England was common.
But the vast majority of pupils who had no previous experience of the language were enjoying it while many schools with no historical commitment to Welsh had invested heavily in recruiting and training teachers.