In the menu bar you can activate AutoSpellCheck by pressing the ABC-Button (F7). Any unknown words will appear underlined in red. To add an unknown word to the standard dictionary right-click it and choose Add › standard.dic. So over time the programme will increase its vocabulary – providing you are always using the same computer of course.
You can add your own personal dictionary under Tools › Options › Language settings › Writing aids. In the window User-defined dictionaries select New, give it a name and choose which language it should apply to. Then press Edit. Type in a new word and confirm each one by pressing New.
The language in use is defined in the paragraph style under the tab Font. So a text containing several languages can be spell checked providing you have the appropriate dictionaries installed. You can download extra languagepacks from openoffice.org (look for Extensions).