Ubuntu is available in a number of major languages. Check them out here:

On XO computers: Balsa-OLPC

Open Applications Menu--System--Language Support. Find the language you wish to install, check the box to the right of the language name. Click Apply. Log out and back in again to register the changes.

Then go back into the Language Support window. Change the Default Language. Make sure "Enable support to enter complex characters" is checked. Click Apply. When it's finished, log out and back in again.

Open Applications Menu-Settings-SCIM Input Method Setup. Under FrontEnd--Global Setup click "Share the same input method among all applications." Leave other settings here alone unless you know what you're doing.

Under IMEngine--KMFL install any Keyman keyboards if you haven't done so. See here for instructions.

On other netbooks: Balsa-Netbook (soon to be available)

Go to Applications Menu--System--Language Support. Click on "Install/Remove Languages." Choose the language(s) you want to install: Click the checkbox to the right of the language name.

Make sure the Components you wish to install for  each language are checked (if greyed out, that component is not available or not needed for the language). Then Apply Changes.

Back in the Language Support window, choose the language you want for users and logins. If your selections for the two windows should be different, uncheck "Keep the same." Make sure the "Use input method engines (IME) to enter complex characters" box is checked.

In Intrepid:

Right click on the bottom panel select 'add new item' and pick 'keyboard Layout Switcher' from the list to place a small icon with the current language that also allows you to switch languages by clicking it.

To add the new keyboard layout  go to the ubuntu menu>settings>settings manager click on the keyboard and then on the layouts tab. Un-check the ‘Use X configuration’ box click the ‘+Add’ button and add your language.

To change the layout with a keyboard shortcut combination you have to go to the terminal (…) and type
nano .xfce4_startup
At the bottom of the file that opens add the line:
setxkbmap -option "grp:shifts_toggle"” to change layout pressing both shift keys or
setxkbmap -option "grp:alt_shift_toggle" to change by Alt+Shift

To change the GUI to another language 

You need, in addition to the packages described at 4 above, to get with synaptic the language-pack-gnome-pl   package  (in my example).  The log out and in the login screen select your interface language either from the left-most button on the bottom bar (‘Options’) or the 3rd from the left (usually marked ‘Last language’)
That’s it!

Keep in mind that changing the settings here will also change the format of time and date, numerical parameters eg decimal point symbol etc. If you want a mixed environment  (eg Polish interface and US decimal point)