Scale typically needs an acid to break it down. The issue you'll face is using an acid that is strong enough to break the scale down, but that also won't damage the chrome of the shower head.
The mildest acid that could work would becleaning vinegar. Get a container where the shower head will fit in, and then fill it with enough vinegar to submerge the scale build-up. Leave the shower head in the vinegar for a few hours, and then with a soft bristle brush, try to dislodge the build-up. You might need to leave the shower head in the solution overnight.
If the above isn't effective, you could consider usingCLR 750ml Bathroom And Kitchen Cleaner. I'd recommend doing a test section on an inconspicuous section to ensure it doesn't affect the chroming.
I would not recommend using Jif or any other abrasive cleaner as it will leave micro-abrasions in the chrome and dull it. - MitchellMc