Answer:

There are two sources of exam entry data. Data covering 16-18 year old school and college students 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 (all schools, colleges and adult entries in England, Wales & Northern Ireland) - NEW, updated on 18 August 2011

• National trends in AS and A level entries 1995 to 2011, (xls, 32kb) across all languages, all candidates  including school, college and adult entries in England, Wales and Northern Ireland (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.
  

Schools and colleges in England (16-18 year old) - REVISED, 12 January 2011:

• AS and A level language entries trend analysis 1996 to 2010 , schools and colleges in England (CILT analysis of DCSF data, pdf, 187kb)
• A level entries and passes by local authorities, 2008/09 for maintained schools only (xls, 66kb)
• Regional trends in A level entries and passes 1997 to 2005/6 for all languages, schools in England (DCSF data, xls, 98kb)
• To explore achievement data and entries of modern languages as compared to other subjects see DfE: GCE/Applied GCE A/AS and Equivalent Examination Results in England, 2009/10 (Revised), January 2011.
  

Schools and colleges in Wales:

• Entries and achievements in AS level and A level, modern foreign languages, Wales, 2003-2008, Welsh Assembly Government. (xls, 52kb)
• Modern foreign languages in schools in Wales, 1999 to 2007, Statistical Directorate, Welsh Assembly Government. (pdf, 190kb)

Scotland:
Higher Grade entries in Scotland 2001 to 2010 (Scottish Qualifications Authority data, all entries, available from Scottish CILT).

Northern Ireland:

A level entries 2008/09 (xls, 25kb), data supplied by the Department of Education, Northern Ireland