OS X El Capitan: Change the language your Mac uses
Although your Mac is set to display the language of the country in which it was purchased, you can choose a different language to use. For example, if you bought your Mac in the United States but you work primarily in French, you can set your Mac to use French.
Choose Apple menu > System Preferences, then click Language & Region.
Do one of the following:
Add a language: Click the Add button , select one or more languages in the list, then click Add.
The list is divided by a separator line. Languages above the line are system languages that are fully supported by OS X and are shown in menus, messages, websites, and more. Languages below the line aren’t fully supported by OS X, but may be supported by apps that you use, and shown in their menus and messages, and on some websites.
If a selected language can’t be typed using the input source that’s currently selected in the Input menu in the menu bar, a list of available input sources is shown. If you don’t add an input source now, you can add the input source later in the Input Sources pane of Keyboard preferences.
Change the primary language: Drag a different preferred language to the top of the Preferred languages list.
If OS X or an app supports the primary language, menus and messages are shown in that language. If it doesn’t, it uses the second language in the list, and so on. The language may also be used on websites that support the language.
You can set other language and region options in Language & Region preferences.