How to format localised strings in Swift?

I am learning to localise my app to Simplified Chinese. I am following this tutorial on how to do this.

Because the tutorial is based on Obj-C, formatted strings can be written like this:

"Yesterday you sold %@ apps" = "Ayer le vendió %@ aplicaciones";

"You like?" = "Es bueno?";

But I am using Swift. And in Swift I don't think you can use %@ to indicate that there is something to be placed there. We have string interpolation right?

My app is kind of related to maths. And I want to display which input(s) is used to compute the result in a detailed label of a table view cell. For example

--------------
1234.5678
From x, y <---- Here is the detailed label
--------------

Here, From x, y means "The result is computed from x and y". I want to translate this to Chinese:

从 x, y 得出

Before, I can just use this:

"From \(someVariable)"

with the strings file:

"From" = "从 得出";

And this is how I would use it in code

"\(NSLocalizedString("From", comment: "")) \(someVariable)"

But if this were used in the Chinese version, the final string will be like this:

"从 得出 x, y"

I mean I can put the 从 and 得出 in two different entries in the strings file. But is there a better way to do it?

You can use %@ in Swift's String(format:...), it can be substituted by a Swift String or any instance of a NSObject subclass. For example, if the Localizable.strings file contains the definition

"From %@, %@" = "从 %@, %@ 得出";

then

let x = 1.2
let y = 2.4
let text = String(format: NSLocalizedString("From %@, %@", comment: ""), "\(x)", "\(y)")
// Or alternatively:
let text = String(format: NSLocalizedString("From %@, %@", comment: ""), NSNumber(double: x), NSNumber(double: y))

produces "从 1.2, 2.4 得出". Another option would be to use the %f format for double floating point numbers:

"From %f, %f" = "从 %f, %f 得出";

with

let text = String(format: NSLocalizedString("From %f, %f", comment: ""), x, y)

See Niklas' answer for an even better solution which localizes the number representation as well.

From WWDC 2017:

let format = NSLocalizedString("%d popular languages", comment:"Number of popular languages")
label.text = String.localizedStringWithFormat(format, popularLanguages.count)

One more simple example

let changeable = "something"
let result = String(format: NSLocalizedString("stringId", comment: ""), arguments: [changeable]) // Hello World and something

localizable.strings with

"stringId" = "Hello World and %@";

• comment parameter doesn't have effect on result and is used for translators and by genstrings code-gen as comment

New in iOS 15 and macOS Monterey you can use the new refined method for String. String(localized: "From (x), (y)", comment: "The result is computed from x and y")

They did a lot of updates in 2021 for localization with Xcode. Check out this video for WWDC21 for more info.

https://developer.apple.com/videos/play/wwdc2021/10221/

In objective C, if we want to get strings added at runtime as below

John Appleseed is the name

YourLocalizable.strings

"theStringToDisplay" = "%@ is the name";

ViewController.m

NSString *username = @"John Appleseed";
NSString *messageBeforeFormat = NSLocalizedStringFromTable(@"theStringToDisplay", @"YourLocalizable", nil);
NSString *messageAfterFormat = [NSString stringWithFormat:messageBeforeFormat, username ];

self.yourLabel.text = messageAfterFormat;

Further explanation in this nice post

https://www.oneskyapp.com/academy/learn-ios-localization/2-format-strings-plurals/