By default HTML Editor use only one dictionary for Spell checking. It's name is 'en-US.dic'.
You can make HTML Editor to use any other dictionaries, that are present in dictionaries folder.
For this purpose you should use inner Dictionaries collection. This collection redefines default dictionary.
Note that too many dictionaries for Spell checking increase checking time.
See also Spell checking.
You can download dictionaries here Spell Checker dictionaries .