Hi,
As part of your script, you can include the following line:
app.execMenuItem("Spelling:Check Spelling");
If you are in a locale that LC Designer supports you are OK. However if you are in a geographical location that is not supported, then you can force LC to map your spell checker to the nearest/closest locale that is supported. For example, Ireland is not supported, so I have made a once off change to a .js file in the LC folders to make the spell checker to the UK.
There is a very good post on this here (http://blogs.adobe.com/lcdesigner/2008/05/spell_check_locale_mapping.html). As I say if you locale is supported you do not need to do this.
Good luck,
Niall