Agreed on excuse. But if the vendor doesn't have people already familiar it might be worth asking whether it's worth paying for them to learn it if you don't already have an existing codebase that requires it.

I've been working in Swift,t he language itself is nice, the tooling and community documentation is a bit rough. For example Cocoapods support for Swift pods is still prerelease and there are a lot of small differences coming from Objective C that can require a little research.

I wouldn't write safety-critical code in either language though.