It's very hard, very ugly and often useless to combine multiple programming languages in 1 file.

It is however possible to have a big project written in more than 1 language. For example, both Mozilla Firefox and MySQL have C and C++ code in them. When it comes to big projects, this practice is often used because a particular language provides some features that another does not. In PHP, for example, you can ask for the execution of a binary executable, take its result and use it in your PHP code from that point on.

If you're curious about what languages are used for some of the most important open source projects in the world, you can check out Ohloh.net. Statistics about the source code of many projects can be found there.