To install a new language in an Odoo instance, follow these steps:

1. Select the Settings|Translations|Load a Translation menu option. In the resulting dialog box, select the language to install from the list of available languages. If Website is installed, you will also be given the option to pick the websites where the language is available:

2. Now, click on the Load button and the language will be installed:

3. New languages can also be installed from the command line. The equivalent command for the preceding steps is as follows:

    $ ./odoo-bin -d mydb --load-language=es_ES

4. To set the language used by a user, go to Settings|Users & Companies|Users and, in the Preferences tab of the User form, set the Language field value. While you're at it, you can use this opportunity to set the user's Timezone:

Users can also set these configurations themselves through the Preferences menu option. This is available when they click on the username at the top-right of the web client window.