If you're allergic to metal can you pierce your tongue?

Generally, no...I mean, if you were allergic to pollen you wouldn't want to have a flower constantly strapped to your face now, would you?

If it's only one specific type of metal, there are many different kinds of metals that are used to pierce. Titanium, silver, gold, or stainless steel, there is a high likelihood that one of those won't cause an allergic reaction. Talk to your piercer about it and see what they have to offer.