What are some situations where languages should be mixed?
I'm not talking about using ASP.NET with C# and HTML or an application written in C accessing a SQL database through SQL queries. I'm talking about things like mixing C++ with Fortran or Ada with Haskell etc. for example.
[EDIT]
First of all: thank you for all your answers.
When I asked this question I had in mind that you always read "every language has its special purpose".
In general, you can get almost everything done in any language by using special libraries. But, if you are interested in learning different languages, why not take the programming language that serves your purpose best instead of a library that solves a problem your language wasn't originally designed for?
Well, the most obvious (and the most common) situation would be when you use some high level language to make most of your program, reaping the benefits of fast development and robustness, while using some lower level language like C or even assembly to gain speed where it is important.
Also, many times it is necessary to interface with other software written in some other language. A good example here are APIs exposed by the operating system - they're usually written with C in mind (though I remember some old MacOS versions using Pascal). If you don't have a native binding for your language-compiler infrastructure, you have to write some interface code to "glue" your program with "the other side".
There are also some domain-specific languages that are tuned specifically to efficiently express some type of computation. You usually don't write your entire program in them, just some parts where it is the appropriate tool. Prolog is a good example.
Last but not least, you sometimes have heaps of old and tested code written in another language at hand, which you could benefit from using. Instead of reinventing the wheel in a new and better language, you may simply want to interface it to your new program. This is probably the most usual (if not the only) case when languages geared for similar uses are mixed together (was that C++ and Fortran you mentioned?).
Generally, different languages have different strengths and weaknesses. You should try to use the appropriate tool for the job at hand. If the benefits from expressing some parts of the program in a different language are greater than the problems this introduces, then it's best to go for it.