Answer:

There are two sources of exam entry data. Data covering school pupils in KS4 only which is released by the DFE annually in October and data covering all candidates which is released by the Joint Council for Qualifications immediately as the results come out in August. Both of these sources can be accessed below.

All data (schools, colleges and adult entries in England, Wales & Northern Ireland) - NEW, updated on 25 August 2011

• National trends in GCSE entries 1995 to 2011, across all languages, all candidates, including school, college and adult entries in England, Wales and Northern Ireland (xls, 25kb) (Joint Council for Qualifications, JCQ, data).
• Separate data for England, Wales and Northern Ireland can be accessed in the downloadable pdf documents on the JCQ website.

School pupil data:

Key Stage 4 entries in England - REVISED, 12 January 2011:

• CILT analysis of DCSF data: GCSE language entries trend analysis, 1994 to 2010, including achievements (pdf, 296kb) 

School entries in Wales:

• Entries and achievements in GCSE in modern foreign languages, all schools in Wales, 2007/08 (pupils aged 15 only, Welsh Assembly Government data)
• Modern foreign languages in schools in Wales, 1999 to 2007, Statistical Directorate, Welsh Assembly Government. (Modern Foreign Languages in Schools)

Scotland:
Standard Grade entries in Scotland 2000 to 2010 (Scottish Qualifications Authority data, all entries, available from Scottish CILT)

Northern Ireland:

GCSE entries by subject (xls, 25kb, data supplied by the Department of Education, Northern Ireland)