IMHO, to use a language, even English, just for the sake of using it would be disasterous in cryptology. I have never heard of these constructive languages except esperanto before. But another language, especially one like esperanto with no official status in any country, in your arsenal to encrypt is golden. Dutch or anyone using these languages would benefit 1000 fold. Any well encrypted message should never be assumed that the message was solely encrypted in the host language. Somewhere down the line it may have been translated into another language, To know another language such as Dutch eluded to, could spell disaster for a hack trying to penetrate the code. I believe that it could only help in assisting the code as long as both creator and cypher of the code know that it is being used. I agree with Aaron with the cycle of translation and using one or more of these languages in there would create difficulty in solving the cypher. Hope I made myself clear.