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Most recent browsers allow you to enable spell checking, along with your language selection, under Settings or preferences. Assuming that is enabled you can make sure that RT 4.4 uses that functionality when composing updates to tickets.
Turn on WYSIWYG browser right-click menu option in RT Preferences:
Now, when entering reply, Mis-spelled words are underlined in red. Right-click while hovered over word, for spelling suggestions:
This has been tested to work correctly with FireFox and Chrome.