Weldmesh fencing is a possibility, comes in galvanised or green pvc
coated, or for higher security the steel mesh stuff they use for
freestanding security/site fencing - basically see what a larger
fencing specialist can do.

You don't say if it's for domestic or business, or if there's room for
the grille to hinge or slide away or needs to be a "lift-off and
store", or if you're looking at this as a daily cycle.

Does it matter if the closed grille is opaque, or do people still need
to see in or out?

Is the purpose to protect the glass against thrown objects or to stop a
reasonably determined entry?

Fencing based grilles will do the former, but can be removed with
boltcutters in a few minutes.

An alternative may be laminated/plate/wired glass (less expensive than
you may expect) or even old-fashioned wooden shutters.