Your choice of preferred language will affect the language of the documentation, the installer and the system in general. Select first the region you are located in, and then the language you speak.
Clicking on the button will allow you to select other languages to be installed on your workstation, thereby installing the language-specific files for system documentation and applications. For example, if you will host users from Spain on your machine, select English as the default language in the tree view and Español in the Advanced section.
Note that you're not limited to choosing a single additional language. You may choose several ones, or even install them all by selecting the All languages box. Selecting support for a language means translations, fonts, spell checkers, etc. for that language will be installed. Additionally, the Use Unicode by default checkbox allows you to force the system to use unicode (UTF-8). Note however that this is an experimental feature. If you select different languages requiring different encoding the unicode support will be installed anyway.