That is a little baffeling. I found that All English except US English does lack the unlearn option but US English works. N.B. my system is in Swedish.
Then I changed Pages 09 to English (US) using tha application Language Switcher and I still could unlearn the word.
All this is on Mountain Lion 10.8.2.
You can delete the word by another route:
1. In Finder press Option when you click on Go in the menu
2. Choose Library
3. Find the folder Spelling
4. Open it and double click on the document named en. It should open in TextEdit.
5. Find the word you want to delete and delte it. Save the document
6. Now it is gone!!
Sorry it shows en_GB
This is what it should look like when you right click on the faulte learned word
