You can check both DSL customization classes and their properties. You can choose which properties of the Customization Elements (Classwith a «Customization» Stereotype) to be spell checked.
You accomplish this by defining a customization class Editing tagged valuecheckSpelling value, found in the properties tag group. By creating a value for this tag, you can choose String properties to check spelling for. By default, there are no properties marked as checkable.