Although you can create non-English characters on a Mac without changing your keyboard layout, some may still prefer to enable a foreign keyboard layout instead. This may be faster than using the keystrokes for non-English characters, and it can help prepare you for foreign keyboards if you are going to study abroad. Here's how to do this:
1. Select System Preferences from the Apple menu in the Finder.
2. Select the icon for Keyboard, as show below.
3. When the Keyboard section loads, select "Input Sources" and then click the ' + " in the lower left corner.
4. When the language selection box pops up, find the language you would like to use from the list, click on it, then click "Add".
That’s it! You can now easily switch between English and German or Spanish keyboard inputs by selecting the preferred language on the menu bar (look for the flags!). Note that Mac OS X switches the language for all applications. Just select U.S. to switch back to English input.
See this article for a discussion of other options for creating non-English characters on your computer.