The question is already answered in Specify Spotlight keywords iOS 9
As you point out, Core Spotlight indexes data in the app. Data that you provide. So all you need to do is provide it with some data so Spotlight Search can find your app.
Adding this to application(_:didFinishLaunchingWithOptions:)
worked for me:
let attributeSet = CSSearchableItemAttributeSet(itemContentType: "application")
attributeSet.title = "French Translator"
attributeSet.keywords = ["french", "translation", "translator"]
attributeSet.contentDescription = "French Language Translation App"
let item = CSSearchableItem(uniqueIdentifier: "5C0B7719-3BE7-4441-892D-A786AA431BDD", domainIdentifier: nil, attributeSet: attributeSet)
CSSearchableIndex.default().indexSearchableItems([item]) {
print("Finished indexing with \(String(describing: $0))")
}