Babel package. When writing in a single language, the simplest way of using it is with uselanguage.

In the preamble, use the following (if your language of choice is Spanish):

uselanguage Spanish

To see a list of available languages, check the Language type.

Languages.

Import the babel package using a given Language.

uselanguage l = usepackage [texy l] babel

If you are using more than one language, consider to use uselanguageconf.

Language configuration. You may use one with uselanguageconf.

Import the label package using a given language configuration, featuring a main language and some others. For example:

uselanguageconf $ LangConf English [German]

This will use English as main language, and German as secondary.

Switch to a given Language.

Use a Language locally.

The function foreignlanguage takes two arguments; the second argument is a phrase to be typeset according to the rules of the language named in its first argument.