"I don't use localizations (probably because there are about 30 language files)" I still didn't understood that.
But I'd go with:


protocol Translations {
    var valid_email: String { get }
}

struct German: Translations {
    var valid_email: String = "Bitte geben Sie eine gültige e-mail Adresse ein"
}

struct English: Translations {
    var valid_email: String = ""
}

extension Locale {
    static func isGerman() -> Bool {
        return Locale.preferredLanguages[0].range(of:"de") != nil
    }

    static func language() -> Translations {
        if isGerman() {
            return German()
        }
        return English()
    }
}

let errorLabel = UILabel()
errorLabel.text = Locale.language().valid_email