My point is that no one should be able to decide whether or a language is a "contrivance" or not. It is a slippery slope that is a dangerous because any language (ie. Chinese) can be deemed contrivance and thus "unethical to teach" using your same logic.

This can be compared to someone teaching their child their own religion. No one should be able to restrict that because religion (and most human belief, pending that it doesn't hurt others or themselves) ought to be outside the bounds of morality.

I fail to see the slippery slope. The whole point of teaching your kid a language, or a set of manners, or religious rituals, or whatever, is to enable him to participate in a community. Raising your child into a fangroup organised around a violent species from a television show is qualitatively different from raising him into a real world minority community with an actual cultural and social life.

The "Klingon community" has no system of helping its members to make friends or find mates, no useful information about the real world, no "payload" of beneficial values to inculcate in its members (as PG put it in "Lies we tell kids"), no inspirational works of art produced by people it recognises as intellectual forebears, etc. That makes it objectively inferior to pretty much any "involuntary" (religious/ethnic/geographical) community no matter how small, and even most "voluntary" communities (e.g. hackers).