It finally works :
$idProduct = 13344;
$_storeId = 2;
$mediaApi = Mage::getModel("catalog/product_attribute_media_api");
$_product = Mage::getSingleton('catalog/product')->setStoreId($_storeId)->load($idProduct);
$items = $mediaApi->items($_product->getId());
foreach($items as $item) {
$item['label'] = 'My cool translation';
$mediaApi->update($_product->getId(), $item['file'],$item, $_storeId);
}
Thanks stackexchange