Does the sight of your neighbor leveling put color in your cheeks? Do you think Americans are meager with their letters?
Queen’s English gives your WoW experience a British flavour, allowing you to appease with fervour that niggling spelling pedant within you. As he or she is wont to do, an American may type "leveling", but to you it will appear as "levelling". Unfortunately it seems too tricky to "fix" chat bubbles, but chat frames and combat frames both work ("Judgement" of Wisdom, anyone?).
To add your own spellings, use the following command:
/qe color colour
...which will replace all occurances of "color" with "colour"
If you don’t like a substitution, use "keep":
/qe keep judgment
...and "judgment" will no longer be replaced with "judgement"
The word list is in the file QEVocab.lua. Suggestions for more spellings are welcome. Queen’s English can also be used (within the scope of single words) to expand abbreviations, eg. "lvl" to "level". Enjoy!