I think it's a bit language-specific. For example, many websites that offer Japanese <-> English lookup use the free EDICT: http://www.csse.monash.edu.au/~jwb/j

I think it's a bit language-specific. For example, many websites that offer Japanese <-> English lookup use the free EDICT: http://www.csse.monash.edu.au/~jwb/j_edict.html http://www.csse.monash.edu.au/~jwb/j_edict.html You could also check out the list of free dictionaries included with a translation app like Lingoes and see if the source data files are available: http://www.lingoes.net/en/dictionary/index.html http://www.lingoes.net/en/dictionary/index.html The Vicon series of dictionaries covers a lot of languages, but I have no idea where they come from. You may also need some logic on a per-language basis if you want to consider conjugation, plurals, etc., since these dictionary files don't list all alternative forms.