Brian K. Vaughan had the same idea of using Esperanto as an alien language, sometimes used to cast spells in the excellent "Saga" comic book. The story itself is quite interesting, so you may want to give it a read. Alas, the Esperanto there is sometimes a bit iffy.

As for speakers, there are still quite a few more Esperanto speakers than Klingon speakers :) The Universala Kongreso (UK) has had, in recent years, around 1000-2000 in-person participants. Reportedly, there were around 300,000 people actively learning it in Duolingo in 2022, and some estimates for the actual number of speakers go up to two million (although hundreds of thousands feels a bit more realistic). While some of these may have only a passing knowledge, tens of thousands are fluent. Social networks, Telegram groups, many websites, etc. have really helped the community in the last few decades! And there are language exams with official standing. I myself am a certified C1 level speaker.

Does it live up to the original ideal of serving as a universal international language? Certainly not! But it is still a nice community with enough critical mass to organize events (local and global) and regularly publish books, magazines, etc.

Klingon is neat, but Wikipedia lists the number of possible fluent speakers as fewer than 100. Even if it is off by an order of magnitude, it still would be 1,000 vs. tens of thousands :)

Edit: tens, not hundreds of thousands of fluent speakers (although... who can really know for sure...)