After years of ignoring TextEdit's insistence that I was spelling words incorrectly, I set off on a Google search to find out how to replace the default American English dictionary with a British English one. However I soon discovered that it was my omission all along - all I needed to do was choose the Edit:Spelling:Spelling menu item and I could change the dictionary to a British English one! However my time was not completely wasted, I discovered a very slick (and free) improved spell checker for OS X named cocoAspell, an OS X implementation of the open source Aspell. Aspell has a wide selection of additional European language dictionaries available for download.
And before anyone asks, the answer is no, I don't know why the Croatian dictionary is ten times the size of the others...