Although there are several dictionaries installed, the only one for which spelling works (i.e., incorrectly spelled words are flagged) is Greek, which I have installed as an extension. (Available from http://extensions.services.openoffice.org/project/dictionary-gr).
Misspellings in other languages (I tried English and French) are not flagged. If I right click on a misspelled Greek word and select "spelling and grammar", the only language available in the "Text language" combo box is Greek. It seems that the built-in dictionaries are ignored.
I am using LibreOffice under the English Version of Windows XP, using the Greek locale. Could it be that only the dictionary for the language of the current locale is loaded?
I just switched to the French locale, and the only dictionary available is still the Greek one, so this is not locale-related.
*** This bug has been marked as a duplicate of bug 37195 ***