How do you respond...
to people who say Esperanto isn't a real language. This logically doesn't even make sense but I still don't know what to say.
The best response is to ask them why they think it's not a real language. Then when they respond (and it's usually a misconception on their part) I then state, "I can see why you would believe that but unfortunately that's not true / incorrect. I can tell you why if you're interested". You don't want to put them on the defensive otherwise they'll dig their heels in. Also, some people just aren't interested in learning new facts or are just out to be trolls and know-alls. This type of stance on your part will let you know pretty quickly what their intentions are.
Well even my multilingual partner said it is useless to spend time on it but I did it anyway. I like the idea behind it and the logic in the grammar and wordbuilding. It was some kind of goodie for me as I learned it just for fun or to reward myself. And how can anything you learn be useless or wasted and if it is just for keeping your mind or brain busy and sharp.
We don't have a definition of what a "real language" is, so it doesn't make sense to argue about this notion, to prove their point and "win" the argument people will just keep moving the goal-posts.
I'd just say that it is real for me, I use it daily and it's part of my life. If you care about me then you could care about this aspect of me. If not, then there's no use arguing about anything.
Maybe you know this already: http://claudepiron.free.fr/articlesenanglais/reactions.htm
:P I was anticipating that ieujwn jumps in and gives a real life example of moving the goal posts. Nice job providing a custom definition of a "real" language to exclude Esperanto but somehow include Interlingua. For all Interlingua speakers out there: I like the language and the idea behind it, I would never want to pit Interlingua against Esperanto. They are both beautiful, and useful, and real in their own way.
To respond again to the original question: some of those people are just trolling you so you need not bother to respond.
Often when someone says that, they are simply trying to get a rise out of you. In that case, if it's really a good natured tease, you can tease the person back: "English is also not a real language either, it's a mishmash of French German Latin and a bit of Yiddish (especially in this sentence). Yet people speak it despite that. How can it be a real language if the greatest poet of the language couldn't spell his name consistently." :) And remember, you do not have to correct everyone on the internet.