It's the language of the document (and quite possibly the default language for documents) not being set to what you actually use and have dictionaries installed for.
See http://www.oooforum.org/forum/viewtopic.phtml?t=32464
Adds ability for users to notify administrators about spelling mistakes on the site.
Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution 3.0 Unported License.