Taken from: The WordPress MU Manual

While U.S English is the default display language for WordPress the software has the built in capacity to be used in any language. This is done by installing the .mo languages translation files in a languages folder in wp-content.

If you want to create or modify a .mo file you need to use poedit and the .po file (refer to Translating WordPress at for more information).

Installing Languages

1. Download the required .mo languages translation file from:

a. Languages for WordPress MU – contains 36 different languages for WordPress MU that have been collated and edited for easy use
b. http://svn.automattic.com/wordpress-i18n/

TIP:
a) Downloading the Languages for WordPress MU is the easier option because it can be time consuming locating the correct files on http://svn.automattic.com/wordpress-i18n/
b) Languages for WordPress MU contain both .mo and .po languages translation files. To add a language to your site you upload the .mo file. The .po files are so you can edit and improve the translations.

2. Unzip the downloaded files to a folder on your hard drive
3. Create a language folder on your hard drive and add all the .mo file to your language folder
4. Launch FileZilla
5. Enter your domain name, your username and password. NOTE: the same username and password that you use to log into your cPanel.
6. Click Quickconnect

7. Locate the languages folder on your computer

8. Open up your wp-content folder for your site (located within your public_html folder)

9. Select the language folder on your computer and drag it into your wp-content folder on your site

10. Dragging the language folder from your computer into wp-content folder of your site uploads these new languages to your WPMU site however they aren’t enabled.

Language Options

There are two options for changing default language on your site:

1. Site Admin > Options - used to change the default language site wide i.e. the language that is displayed inside the dashboards of all blogs site wide and on the login screen.
2. Settings > General – used by your users to override your default language setting. This enables your users to set their own preferred default language.

For example, here is what it would look like if its default language was set to Dutch

Changing the sitewide language

1. Click on Site Admin > Options
2. Select the default language
3. Then click Update Options

Changing language on a per blog basis

Users can override the default language settings and set their own preferred default language using Settings > General.

1. Click on Setting > General
2. Select the default language under blog language
3. Then click Save Changes