Battery Leak

Yeah, get as much of the dry deposits off, without applying loads of pressure and then a little vinegar on a cotton bud and give it a good clean. Then sit for an hour to dry off.

Should work, make sure its fully dry, then put some new batteries in and it "should" work again.

All you can do is try to clean them as best as you can, electronic circuits are safe to clean with a cotton bud, as long as the circuit is dry when powered up. Corroded contacts probably won't be recovered to their former glory so easily, use a cotton bud or baby wipe to remove as much of the battery deposit as you can (use gloves the alkali used can react with oils and fats in your skin).

Acid or alkaline?

There are easy ways to clean them and hopefully recovery your lights in working order.

Firstly you'll want to get rid of anything that easily flaked off, for that, use an old soft toothbrush

If its acid you want to use baking soda on a cotton bud

Alkaline you want to use lemon juice or vinegar, again on a cotton bud.