OS X is localized to a number of languages, and with a few settings changes, you can set the OS interface -- as well as many programs -- to display in a different language. This is handy for multilingual types, but if you have inadvertently changed it to a language you do not understand, you might find it quite difficult to revert.
Fortunately, while the languages change, the layout still stays essentially the same. To switch back to your preferred language, you will need to follow these steps:
First go to the system preferences by opening the Apple menu and selecting the fourth menu item (it should be immediately under the first separator bar).
Next, select the Language & Text system preferences, which looks like a blue United Nations flag and should be in the top row and fifth from the left in Mountain Lion (its positioning may be slightly different in other versions of OS X).
In the Language & Text system preferences, select the first tab, where you will see a lists of languages. Locate your preferred language and move it to the top of the list.
Next locate the Region tab, which may be called "Formats." In Mountain Lion this is the third tab in from the left, but again this may be different in other versions of OS X. An alternative way to find this is to click through the tabs sequentially until you see the one with date and time formatting options. When found, choose your preferred country from the top-most menu.
When finished, log out and back in (or restart your computer); the system should now be in the appropriate language. You can then navigate back to the system preferences to make finer adjustments for keyboard layouts, specific date-and-time formatting, and other details regarding different language preferences.
If you are having difficulty navigating, another option is to use the OS X Terminal to clear these settings. This can be done by opening the Spotlight menu and typing "Terminal" to locate and open it, or by opening the Applications folder from the Go menu in the Finder (you can also press Shift-Command-A to do this) and then opening the Utilities menu from there. You can then locate Terminal by its icon.
In Terminal, run the following command to delete the hidden global preferences file that holds your language settings, and then log out and log back into your user account:
defaults delete -g