As mentioned elsewhere here, Ocaml and the ML family are often cited as easy to implement other languages, due to pattern matching, enum variants, sum types, etc. Since newer languages like Rust and Swift copied those features, you might be more interested in them since they're more popular than older ones like Ocaml and Haskell. Personally I like the syntax sugar in Swift.

Also I found it useful to compare other people's implementations to get a feel for different approaches. See this suggestion: https://news.ycombinator.com/item?id=28479120