I propose to add a boolean property to GtkTextTag for the spell checking, to be able to disable the spell checking in certain regions.

By convention, the spell checking would be enabled in all the buffer, and the tag should be applied to the regions where we don't want the spell checking. If the application wants the opposite, it's not difficult to apply the negation.

The API addition:
The GtkTextTag:no-spell-check boolean property. If the property is false, it has no effects.

I think a convention is needed, because the users of this tag can be independent. For example GtkSourceView can apply this tag, and GtkSpell can read the tag, without dependencies between the two libraries.

And if one day the spell checking is implemented in GTK+, it is one thing that would anyway be needed (in this manner or another).