I guess it depends on how much past experience they had with an array-based, stack-based, logic-based, pure functional or lisp family language, and how long ago

I guess it depends on how much past experience they had with an array-based, stack-based, logic-based, pure functional or lisp family language, and how long ago that was. I can believe an experienced engineer can learn the basics of any language in a day. But being proficient and idiomatic is another matter.