I have localised info.plist file but how can I change info.plist localisation language in swift?

What does "in Swift" mean here?

If you're asking if you can write Swift code to modify the info.plist file at runtime, then the answer is no.

If you're asking if you can write Swift code to activate a different language/localization at runtime, then the answer is still no, because languages are switched system-wide by the user. Apps can't change their own language.

If you're asking a different question, could you explain in a bit more detail?

Apps can't change their own language.

Yes, unfortunately.