Mac OS has long provided a language translation facility, first via Sherlock and, starting with OS X 10.4, the Translation Widget in Dashboard. In both cases there is no translation application on your machine: Sherlock and the Widget are simply interfaces to an online translation service, in this case Systran, which can also be accessed via any web browser.
Anyone who uses this to translate foreign text into his native language will quickly see the limitations. Relying on it to translate your native language into one you don't yourself know, in the expectation of making yourself correctly understood, is not realistic. The reasons for that lie in the inherent shortcomings of Machine Translation. Only humans can really do it right.
For alternatives to what comes with OS X, there is a good list here. Some worth checking out are LiveDictionary, WordLookup, TranslateIt, Google's Language Tools, and Langenburg.com, which is a gateway to multiple services.
For translation software that runs on your Mac rather than via the Internet, there is NeuroTran and InteractiveTran. For your iPod, you can try iLingo.