It might be useful to change the keyboard input language, especially when you need to type text in a foreign language. The Input menu of Mac OS X (displayed at the top left corner of the menu) allows you to rapidly switch between different languages. This tutorial explains how to define additional keyboard languages in a couple of easy steps:
Click on the Apple menu > System Preferences and then on Keyboard. In the Keyboard tab, make sure that the Show Keyboard & Character Viewers in menu bar checkbox is ticked to be able to launch onscreen keyboard viewer from the Input menu.
The next step is to go to the Input Sources tab. Click on the + button to get a list of languages to choose from (you can also make a search by country). Once you have made your selection click on Add button. Tick the Show input menu in menu bar checkbox and flag icon corresponding to the selected language will be added to the Input menu:
That's it; you can now switch to input language of your selection by clicking on the country icon. The new setting automatically applies to both the physical and onscreen keyboard.