How to change the OS X system language
Adjusting OS X to use your preferred languages only takes a few adjustments.
Like most operating systems, OS X supports multiple languages, with full character sets and fonts for practically every language. In addition, there is full localization for a number of these languages, so you can have the system display menus and other system features in the language of your choice.
To do this, open the Language & Text system preferences, and in the Language tab drag the language you would prefer to the top of the list. After a restart, your system and applications will use that language, if they include localization support for it. Be aware of that qualifier because in many situations you might not see any results of this change.
If for example you enable Quechua in the list and drag it to the top, since the OS unfortunately does not contain localization for that language, it will resort to the next available language on the list for menus and other content. However, if you have a special third-party application that has localization support for Quechua, then putting Quechua above English in the list will ensure that that application primarily displays its Quechua translation.
This approach to languages in OS X is conveniently managed in the one system preferences pane; however, in older versions of OS X Apple includes additional options for managing languages on the application level. By getting information on the application itself (select it and press Command-I), you will see a language section where the available localizations in the program are listed. In here you can enable or disable specific localizations for this program.
Keep in mind that if you change your system's language in the Language tab, this will only change the text that is displayed, and not alter input methods and date formats that are commonly used for those languages.
Therefore, be sure to also adjust these settings (for dates, numbers, currencies, and times) in the Region tab, and go to the Input Sources tab to enable the character viewer and supply alternative keyboard layouts in the system's input menu.