In the following applet there is some incorrect text with the typical highlighting of the wrongly spelled words. You can right-click these wrongly spelled words to get a suggestion on how to correct them (please left-click into the applet first to activate it). In the right-click popup menu you will also find a configuration entry for the language. The menu and its functionality is fully provided by the JOrtho library.
Another option is to press the F7 key. You will then get a new window with suggestions from the dictionary and the option to add new words into the user-dictionary (which is deactivated for this web-page due to security reasons).