You need to produce code that not just runs, but is idiomatic in the target language. I'm not aware of ANY transpilers that do this for target and source langua

You need to produce code that not just runs, but is idiomatic in the target language. I'm not aware of ANY transpilers that do this for target and source languages that run in different runtimes (i.e., not coffeeacript to JS). You need every language feature to translate cleanly, and that means that a translation failure makes it impossible to convert a logical unit (module, package, etc) at once. Simply having a translated version of the code without making it idiomatic means it's impossible to read or modify.