I'm afraid I am having a hard time understanding your setup and the issue you are facing.
The internalization feature provided by RadRichTextBox's spellchecking functionality depends on the current culture, i.e. in order to make sure the added by you English / British dictionary is used you can specify the current culture:
Alternatively you can specify the spellchecking culture of a specific control spell checker like this:
Please let me know if I misunderstood you or you need further assistance.
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