Just over a third of Welsh 15-year-olds took at least one GCSE in 2003, compared to 46 per cent in 1996.
Total entries have fallen from more than 19,500 in 1995 to around 14,000 last year, although A and AS-level entries have increased slightly.
However, pass rates at all levels are down, and languages are increasingly the domain of girls: they accounted for 62 per cent of GCSE and 75 per cent of A-level entries in 2003.
More than twice as many English children as Welsh took a modern foreign language GCSE last year (73 to 34 per cent). But languages teachers across the border fear their subjects will suffer a similar decline in popularity from September, when they will no longer be compulsory after age 14 - as already is the case in Wales.
A Welsh Assembly spokeswoman said pupils see languages as "difficult" and are therefore more likely to drop them at 14.
Ceri James, director of CiLT Cymru, the language centre in charge of delivering Languages Count, the Assembly's strategy for boosting the subject, agrees.
"What we are trying to do in our training is to encourage teachers to get pupils through the curriculum levels more quickly in KS3, so they get to use tenses more quickly and feel they can manipulate the language better.
"If they can get higher levels at KS3, they might feel 'I'm doing OK and can carry on with languages'."