Pascal is still very popular for teaching programming in post-soviet countries.

Rules for languages differ by contest. I believe all of them accept C and C++ solutions, most also accept java. Erlangists and Lispers are going to have a bad time, though.

TopCoder, ICPC and IOI all run your code on their machines, and are limited to a small set of languages (C, C++ and Pascal for IOI; C, C++ and Java for ICPC). Others like FB Hacker Cup and Google Code Jam supply the test data, and you just submit the output file; these allow any language.

There is one site (http://spoj.pl) that takes the former approach but supports a huge variety of languages from assembler to Haskell.

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).

I know that most of them allow Pascal, C and C++. Some of them alllow Java.

I coded on Pascal myself when I was of the winners of Ukranian computer science olympiad.