Can I check if a string is an existing word?

If this is MacOS, use NSSpellChecker

https://developer.apple.com/documentation/appkit/nsspellchecker

For IOS, use UITextChecker;

func isCorrect(word: String) -> Bool {
    let checker = UITextChecker()
    let range = NSRange(location: 0, length: word.utf16.count)
    let misspelledRange = checker.rangeOfMisspelledWord(in: word, range: range, startingAt: 0, wrap: false, language: "en")
    return misspelledRange.location == NSNotFound
}

See complete explanation here:

h ttps://www.hackingwithswift.com/example-code/uikit/how-to-check-a-string-is-spelled-correctly-using-uitextchecker

tested in IOS playground :

let tested = isCorrect(word: "correct")

true

let tested = isCorrect(word: "co rrect")

false

It works nearly perfect, but it does not know some German nouns, maybe because they are not written with an upper cased letter. Is there any way to check if the word exists by ignoring if the word has an upper or lower cased letter?

you need to check for an existent word by comparing

save the words and them compare

// for save
UserDefaults.Standard.set((wordToCompare), forKey: "(namedasyoulike)")

to call 
UserDefaults.Standard.string(forKey: "(storekey)")

compare 

let compare1 = UserDefaults.Standard.string(forKey: "(storekey)")
let compare2 = UserDefaults.Standard.string(forKey: "(storekey2)")

them if compare1 == comapre2
{
print(words equal)
} else {
//what ever you want do
}