Phrase.swift

class Phrase {
    
    // MARK: -
    // MARK: Properties
    
    private let template: String
    
    private var lookup: [String: String] = [:]
    
    // MARK: -
    // MARK: Initialization
    
    init(template: String) {
        self.template = template
    }
    
    // MARK: -
    // MARK: Public
    
    func put(key: String, value: String) -> Phrase {
        lookup[key] = value
        return self
    }
    
    func format() -> String {
        var formatted = template
        
        for (key, value) in lookup {
            formatted = formatted.replacingOccurrences(of: tokenize(key: key), with: value)
        }
        
        return formatted
    }
    
    // MARK: -
    // MARK: Private
    
    private func tokenize(key: String) -> String {
        return "{" + key + "}"
    }
    
}

Example Usage:

let template = "Hi {first_name}, you are {age} uears old."
let formatted = Phrase(template: template)
    .put(key: "first_name", value: "Cody Robertson")
    .put(key: "age", value: "23")
    .format()

print(template)   // prints: "Hi {first_name}, you are {age} uears old."
print(formatted) // prints: "Hi Cody Robertson, you are 23 uears old."