let descriptionString = String(format: "localised_key".localized(), Harsh, 10)
let description = NSMutableAttributedString(string: descriptionString, attributes: [NSAttributedString.Key.font: UIFont(name: "NotoSans-Regular", size: 15.7)!, NSAttributedString.Key.foregroundColor: UIColor(rgb: 0x000b38), NSAttributedString.Key.kern: 0.5])
let rangeName = descriptionString.range(of: "Harsh")
let rangeClass = descriptionString.range(of: "10")
let nsrangeName = NSRange(rangeName!, in: descriptionString)
let nsrangeClass = NSRange(rangeClass!, in: descriptionString)
description.addAttributes([NSAttributedString.Key.font: UIFont(name: "NotoSans-Bold", size: 15.7)!, NSAttributedString.Key.foregroundColor: UIColor(rgb: 0x000b38), NSAttributedString.Key.kern: 0.5], range: nsrangeName)
description.addAttributes([NSAttributedString.Key.font: UIFont(name: "NotoSans-Bold", size: 15.7)!, NSAttributedString.Key.foregroundColor: UIColor(rgb: 0x000b38), NSAttributedString.Key.kern: 0.5], range: nsrangeClass)

for more references, use this

let withFormat = "my name is %@ and i study in class %@"

There are different ways to do so, but in my opinion, one of the easiest way would be to use tags:

Use tags around the placeholders (and other parts if needed):

let withFormat = "my name is <b>%@</b> and i study in class <b>%@</b>"
let withFormat = "my name is [b]%@[/b] and i study in class [b]%@[/b]"
let withFormat = "my name is **%@** and i study in class **%@**"

Tags can be HTML, Markdown, BBCode, or any custom you'd like, then, replace the placeholder values:

let localized = String(format: withFormat, value1, value2)

Now, depending on how you want to do it, or which tag you used, you can use the init of NSAttributedString from HTML, Markdown, etc, or simply using NSAttributedString(string: localized), look yourself for the tags and apply the render effect needed.

Here's a little example:

let tv = UITextView(frame: CGRect(x: 0, y: 0, width: 300, height: 130))
tv.backgroundColor = .orange

let attributedString = NSMutableAttributedString()

let htmled = String(format: "my name is <b>%@</b> and i study in class <b>%@</b>", arguments: ["Alice", "Wonderlands"])
let markdowned = String(format: "my name is **%@** and i study in class **%@**", arguments: ["Alice", "Wonderlands"])
let bbcoded = String(format: "my name is [b]%@[/b] and i study in class [b]%@[/b]", arguments: ["Alice", "Wonderlands"])

let separator = NSAttributedString(string: "\n\n")
let html = try! NSAttributedString(data: Data(htmled.utf8), options: [.documentType : NSAttributedString.DocumentType.html], documentAttributes: nil)
attributedString.append(html)
attributedString.append(separator)

let markdown = try! NSAttributedString(markdown: markdowned, baseURL: nil) //iO15+
attributedString.append(markdown)
attributedString.append(separator)

let bbcode = NSMutableAttributedString(string: bbcoded)
let regex = try! NSRegularExpression(pattern: "\\[b\\](.*?)\\[\\/b\\]", options: [])
let matches = regex.matches(in: bbcode.string, options: [], range: NSRange(location: 0, length: bbcode.length))
let boldEffect: [NSAttributedString.Key: Any] = [.font: UIFont.boldSystemFont(ofSize: 12)]
//We use reversed() because if you replace the first one, you'll remove [b] and [/b], meaning that the other ranges will be affected, so the trick is to start from the end
matches.reversed().forEach { aMatch in
    let valueRange = aMatch.range(at: 1) //We use the regex group
    let replacement = NSAttributedString(string: bbcode.attributedSubstring(from: valueRange).string, attributes: boldEffect)
    bbcode.replaceCharacters(in: aMatch.range, with: replacement)
}
attributedString.append(bbcode)

tv.attributedText = attributedString

Output: