ICPC restricts you to C, C++ or Java. Most contestants I know use a dirty mixture of C with STL containers. Java often gets a little relaxed time and memory limits (although it doesn't really matter as the intended solution doesn't really hinge on the relative speed difference of languages but algorithms).
Many popular and large contests allow any language you want. This is often solved by letting the contestant download a problem file, running his solution and uploading a solution file along with the source code for verification (cf. Google Code Jam). This frees the organisers from having to support every language, interpreter or compiler anyone would perhaps use (which is often the largest problem when it comes to supporting different languages).
Many popular and large contests allow any language you want. This is often solved by letting the contestant download a problem file, running his solution and uploading a solution file along with the source code for verification (cf. Google Code Jam). This frees the organisers from having to support every language, interpreter or compiler anyone would perhaps use (which is often the largest problem when it comes to supporting different languages).