I just installed Ubuntu 12.04 and... I've got a problem :) I'm living in Poland, so I want to write texts in polish with polish chars such as ąćęłńóśźż, too. Unfortunately, it's impossible. We write polish chars by pressing AltGr and chosen key (e.g.: AltGr + A = ą, AltGr + X = ź, AltGr + C = ć, etc.), but when I press AltGr, it isn't like pressing AltGr, it's like pressing Alt - I can open menus with their mnemonics, and I can't enter polish chars. I've got, of course keyboard layout set to Polish.
And btw. - in Oneiric there was everything OK...
So my question is: how to fix it?
Open "System Settings" and then "Keyboard Layout". Check that there is a Polish layout in the list of keyboard layouts on the left of the window. If there isn't, click the "+" button at the bottom left and add it.
If you have more than one keyboard layout added, you also need to check that you're currently using the right one. When more than one keyboard layout is added, a keyboard symbol with a two letter country code should appear in the bar at the top of the screen. Click this icon and select the polish layout. The icon in the bar should then show the letters "pl" next to the keyboard symbol.
By adding a polish layout to my settings, I can type ą, ź and ć without a problem.
From the "Keyboard Layout" settings page, you can also check that the selected layout is as you expect. With the polish keyboard layout selected, click the small keyboard icon at the bottom left of the window. This should show you a diagram of your keyboard showing which letter can be typed with which key.
Check that the Polish characters are where you expect them to be and that the AltGr key is labelled something like "Level3 ..."
If you are just interested in writing Polish letters on websites, use this Firefox extension tool: https://addons.mozilla.org/en-US/firefox/addon/polski-domek-toolbar/
If you already have installed Polish language pack, try Super+Shift+Space or Super+Space. This would switch current keyboard layout if there's more than one installed.