Python — via GObject-introspection typelibs. To use them, it is sufficient to import the corresponding module as follows
from gi.repository import GtkSpell
Vala — via GObject-introspection generated VAPIs.
To compile a GTK+3 Vala application which uses GtkSpell, you need to add
--pkg gtkspell3-3.0
to the valac
compiler flags, i.e.
valac myprog.vala --pkg gtkspell3-3.0 --pkg gtk+-3.0
For GTK+2 applications, you would write
valac myprog.vala --pkg gtkspell3-2.0 --pkg gtk+-2.0