Something I've always wanted to do in Java : union of enums. Here, applied to a simple use case : exhaustive (checked by compiler) i18n of an app, with keys coming from different enums.

i18nUsecase.ceylon

// enum 1 of application i18n keys
abstract class AppKeyi18n()
        of title | description {}
 
object title extends AppKeyi18n() {}
object description extends AppKeyi18n() {}

// enum 2 of user i18n keys
abstract class UserKeyi18n()
		of fieldText1 | fieldText2 | fieldText3 {}

object fieldText1 extends UserKeyi18n() {}
object fieldText2 extends UserKeyi18n() {}
object fieldText3 extends UserKeyi18n() {}

// union of enums
alias AllKeyi18n => AppKeyi18n|UserKeyi18n;

// french translation of all keys
String translateFrench(AllKeyi18n key) {
    switch (key) // exhaustive switch
    case (title) { return "frtitle"; }
    case (description) { return "frdescr"; }
    case (fieldText1) { return "frtext1"; }
    case (fieldText2) { return "frtext2"; }
    case (fieldText3) { return "frtext3"; }
}

// english translation of all keys
String translateEnglish(AllKeyi18n key) {
	switch (key) // exhaustive switch
	case (title) { return "entitle"; }
	case (description) { return "endescr"; }
	case (fieldText1) { return "entext1"; }
	case (fieldText2) { return "entext2"; }
	case (fieldText3) { return "entext3"; }
}