My experience is with the Canadian version of the extension posted, as of 2013-01-18, at http://extensions.libreoffice.org/extension-center/american-british-canadian-spelling-hyphen-thesaurus-dictionaries/folder_contents
The spell check fails to detect American spellings as incorrect. The extension that fails is kpp-canadian-english-dictionary-674277-word-list.oxt
This is fixed by removing the noted extension, deleting the libreoffice config (there’s a way to remove just the dictionary pollution, but I don’t know it)
rmdir -rf ~/.config/libreoffice
I installed an earlier version of the extension I had around named Canadian_English_spellchecking_hyphenation_thesaurus–en_CA_2_0_0____2008-07-02.oxt
Spell checker now works as desired.
The extension does not appear to be findable with google. I did find a reference to it in the DVD listing from libreoffice.
I will leave it up on dropbox for a couple of weeks or so.
the technical bits:
“Ubuntu 10.04.4 LTS” 2.6.32-45-generic #102-Ubuntu SMP Wed Jan 2 22:38:04 UTC 2013 x86_64 GNU/Linux
LibreOffice 126.96.36.199 Build ID: 350m1(Build:2)