There finally is a way to do it, specifying multiple configurations in the command line:
ng build --configuration=en,production
Relevant issue in Angular repo
Note that --prod flag is ignored when you use --configuration (so you need to add production to the configuration list explicitly).
Angular docs for --configuration=configuration:
A named build target, as specified in the “configurations” section of angular.json. Each named target is accompanied by a configuration of option defaults for that target. Setting this explicitly overrides the “–prod” flag
Aliases: -c