The warning follows figures that show a lack of progress in boosting the numbers of native language speaking staff in schools.
Now the British Funding Council has called for funding for the assistants to be ring-fenced in an attempt to improve the uptake.
Councils have employed only 70 foreign language assistants for the 2013/14 school year despite a campaign to increase their use.
Before the onset of council cuts, Scottish schools employed hundreds of the assistants - native speaking staff who work to bring language learning alive, as well as fine-tuning comprehension and pronunciation.
The foreign language assistant programme, run by British Council Scotland, is funded by the Scottish Government, but the money is no longer specifically set aside and most councils have decided to use funding elsewhere.
Lloyd Anderson, director of the British Council Scotland, said: "While the number of foreign language staff in Scottish schools has held at 70 for the next academic year, we are still more than 70% down on the numbers reached seven years ago.
"We will continue to push for as many language learners as possible to get access to the authentic linguistic and cultural knowledge that only a native speaker can bring.
"The assistants bring a cultural dimension to language learning that enthuses and inspires young people to take their study of languages to the highest levels. It is time to ring-fence the money to ensure that happens."
The concerns come after a historic decline in the number of pupils taking modern language Higher exams.
The demise of languages in secondary has been blamed on many schools no longer seeing languages as compulsory, despite school inspectors calling for them to be a "core element" in the first three years of secondary.
There have also been problems in primary, with The Herald revealing three-quarters of schools were missing recommended targets for the delivery of modern languages.
Meanwhile, a survey shows a lack of language and cultural skills has had a clear impact on the enjoyment of the UK public when they go on holiday overseas.
A study commissioned by the British Council found 81% of those questioned said they could not speak a foreign language to a high standard, with 35% saying this caused them embarrassment on holiday and 5% admitting it had caused offence.
In addition, 20% said they had paid "over the odds" as a result of not being able to speak the local language. More than half said they ordered food by pointing at menu items to avoid trying to pronounce foreign words.
The research, carried out by Populus among 2000 UK adults, was commissioned by the British Council as part of its work to build relationships for the UK around the world through language, culture and education and advocate the learning of modern foreign languages in the UK.