The RadHtmlField control is the correct choice for your scenario. You can set the Language property from the editor's configuration file - ConfigFile.xml. To add spell checking support to the field control, open the editor tools file - ToolsFile.xml and add the spell checker tool. If you are using the 5.x version of the RadEditor for MOSS, you will also need to copy the resource files from the editor resources folder to ~/App_GlobalResources/ in your MOSS web application.
Here is a link to the RadEditor for MOSS 4.x
documentation, which shows how to add/remove tools from the editor toolbar: http://www.telerik.com/help/moss/editor/removing tools from the toolbar.html
The same help article, but for the RadEditor for MOSS 5.x
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