Unlike Windows, it is very easy to change the language on your Mac.  If you prefer to use your Mac in a language other than English, you should go to the Apple Menu, select System Preferences and then locate the Language & Text  panel.

languagesIn the first tab, which is marked "Languages", you will see a list of the available languages on your system.  Simply drag the languages into the order that you prefer, putting the language that you want to be the main language for your system at the top of the list.

You can also use the "Region" tab to set your region, so for example if you wanted the language to be Spanish but the Region to be UK, using the standard UK layout for times, decimal points and currencies, you would select United Kingdom from the Region tab.

Now all you have to do is logout of your account and log back in again, or restart your Mac if you prefer, and the language will have changed just like that!  The beautiful thing about Mac apps is that the majority of them also change language according to the system language that you've set, so you most likely will now find all your apps in that language too!