//First, define a custom bundle class as like as below

import Foundation

//A key used for exchanging associated object
var _BUNDLE_KEY = 0

class BundleEx : Bundle {

    override func localizedString(forKey key: String, value: String?, table tableName: String?) -> String {

        let bundle : Bundle? = objc_getAssociatedObject(self, &_BUNDLE_KEY) as? Bundle

        return bundle != nil ? bundle!.localizedString(forKey: key, value: value, table: tableName) : super.localizedString(forKey: key, value: value, table: tableName)
    }
}

//Second, in App delegate.application(_ : didFinishLaunchingWithOptions:) try to swizzle the Bundle.main.
//This snippet only runs for the first application execution. In further executions it won't be executed since the langs array will contain
// "fa" language

let langs = UserDefaults.standard.value(forKey: "AppleLanguages") as? [String]

if langs == nil || !langs!.contains("fa") {

    UserDefaults.standard.set(["fa-IR", "fa", "en"], forKey: "AppleLanguages" )
    UserDefaults.standard.synchronize()

    object_setClass(Bundle.main, BundleEx.self)

    let bundle = Bundle(path: Bundle.main.path(forResource: "fa-IR", ofType: "lproj")!)
    objc_setAssociatedObject(Bundle.main, &_BUNDLE_KEY, bundle, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)

    //Changes the direction of all views. You omit it if the determined language is LTR
    UIView.appearance().semanticContentAttribute = .forceRightToLeft

    let sb = UIStoryboard(name: "Main", bundle: Bundle.main)
    let vc = sb.instantiateInitialViewController()
    self.window?.rootViewController = vc
}