I live in a very old appartment which has a food pantry in the kitchen. The pantry has a wall outside and on that wall is a hole.
What do I have to put in that hole in order to match with the pantry requirements?
I am not aware of any venting requirements for a pantry - certainly not for any opening outside. If I were you I would close up the hole and seal it properly. For air exchange in the pantry, I'd put two vents in the pantry door - one low, one high, to allow air transfer.
I'm tempted to say that venting a pantry outside could be a bad idea as I imagine it would be a very attractive smell to rodents who will find their way in. You'd also be forcing conditioned air outside for no real reason which will just increase utility costs.
As suggested in the answer by The Evil Greenbo, two passive vents would probably do the trick just to get the air circulating, and then you could fill the exterior hole.
If you need more airflow you could try a fan in the room, or even just leaving the door open a bit.