How should I handle localization of string which contains multiple variables and some of them should take care of plural

For example:

let count: Int = ...
let unit: Calendar.Component = ...  //.year, .month, .day
let str = "\(count) \(unit) later"

So str could be one of these:

//In English
1 year later
3 months later

//In Chinese（Note: The spaces between the words are being removed.)
1年后
3个月后

What's the best way to localize this string? If the unit part is removed, I know how to do that with stringsdict file, but now I am confused.

The use of stringsdict works just fine for this case. You just need an entry for each possible unit.

The English setup would include something like:

<key>years later</key>
<dict>
    <key>NSStringLocalizedFormatKey</key>
    <string>%#@years@</string>
    <key>years</key>
    <dict>
        <key>NSStringFormatSpecTypeKey</key>
        <string>NSStringPluralRuleType</string>
        <key>NSStringFormatValueTypeKey</key>
        <string>d</string>
        <key>one</key>
        <string>%d year later</string>
        <key>other</key>
        <string>%d years later</string>
    </dict>
</dict>
<key>months later</key>
<dict>
    <key>NSStringLocalizedFormatKey</key>
    <string>%#@months@</string>
    <key>months</key>
    <dict>
        <key>NSStringFormatSpecTypeKey</key>
        <string>NSStringPluralRuleType</string>
        <key>NSStringFormatValueTypeKey</key>
        <string>d</string>
        <key>one</key>
        <string>%d month later</string>
        <key>other</key>
        <string>%d months later</string>
    </dict>
</dict>
<key>days later</key>
<dict>
    <key>NSStringLocalizedFormatKey</key>
    <string>%#@days@</string>
    <key>days</key>
    <dict>
        <key>NSStringFormatSpecTypeKey</key>
        <string>NSStringPluralRuleType</string>
        <key>NSStringFormatValueTypeKey</key>
        <string>d</string>
        <key>one</key>
        <string>%d day later</string>
        <key>other</key>
        <string>%d days later</string>
    </dict>
</dict>

Then your code becomes something like:

let count: Int = ...
let unit: Calendar.Component = ...  //.year, .month, .day
var key: String?
switch unit {
    case .year:  key = "years later"
    case .month: key = "months later"
    case .day:   key = "days later"
    default: break
}
if let key {
    let fmt = NSLocalizedString(key)
    let str = String(format: fmt, count)
}

If you might have a bunch of these, it might help to add an extension to Calendar.Component to create a string from a unit so you don't need to scatter the clumsy switch statements everywhere.