… then first pretend to show interest in the language s/he creates and after a lengthy explanation say that you were “just kidding”.
Second on the level of annoyance is to misunderstand examples s/he makes. I showed someone the longest rejistanian word to explain agglutination as well as the fact that verbs can indicate more than number and tense. These people did not get it. I now realize that they apparently thought that it was just a code and there was no reason why a complex sentence like “It is possible albeit not likely that they won’t get elected” has a code word (the translation is: Min’ki’lanjamesit’rala’xetsukovomin’ta, eg: they-FUT-SUBJ4-PASS-elect-NEG where “elect” is composed of ‘xetsu meaning to choose and kovomin’het meaning democratic government). But at that time, I had a very hard time not to think that it was done on purpose.
As third option, just ask: “what is it good for?” I could explain in great detail how it helps me to think more clearly about language, how it helped my English, how I feel better when immersing into my conworld, how it is calming to work on it, but instead, I guess another answer would be better: “What do you do in your free time?” and when an answer is given: “What is that good for?” Think for a moment about how insulting this is if done with more mundane hobbies: “I like to listen to ambient music*” – “What is that good for?” See my point? The people around me do many things, which are utterly boring to me, but highly satisfying to them. Implying that these things are useless would mean to attempt to impose my perception on them and that is an utter sign of disrespect of their own sapience.
An extra bonus cruelty: If you are the boy/girlfriend of a conlanger tell her/him that you will divorce her/him if your hypothetical child’s first words are in the conlang.
I guess what I should have learned by now is that the mundanes have no idea what conlanging is. They also have often no intention to learn.
*you cannot believe how lekie that genre is to me at the moment.