I will say that a huge advantage Esperanto has going for it is the number of speakers. Online, you can find many communities. Once you’re in the community, you can literally couch surf the world, staying with fellow Esperantists. There’s a huge corpus of reading material (I have a physical edition of The Hobbit in Esperanto), and even a few movies. It pops up here and there in popular culture, too; the name of the watch company Movado was actually taken from Esperanto.

Being popular means having chances to use the language. Not just study, and play by yourself, but interact with other people. I lurk in Ido and Lojban chat rooms, and they’re mostly dead; Esperanto chat groups tend to be active.

The biggest thing, for me, is when a conlang is used for something other than just discussing the conlang itself. Esperanto has that going for it, and so it’s always the conlang I return to. No, it’s not perfect. Yes, it has some internal politics and debate, most lately especially around gender issues. But it has a certain momentum, and if you’re looking for something you can use, Esperanto is the best choice (IMHO).

Edit 2023-07-02 I forgot to mention! There’s an entire Mastodon instance dedicated to Esperanto; you’re expected to post in Esperanto: https://Esperanto mast on host.

Obviously, a better choice would be a living language. Chinese or Hindi give you the most bang for your buck. But if you like traveling to Germany, learn German.

Heck, learning Latin is a fantastic use of time! It gives you a foot in the door to a half-dozen popular languages - enough to literally make yourself understood in a pinch.

For the internet, and if your interest is conlangs, I’d support your pick of Esperanto. Languages have the most value when there are other people to talk to with them, and Esperanto is the conlang with the most speakers, in the world, by far. Plus, it’s a nice language, and will teach you a lot about languages in general. I think it’s a great teaching aid for parts of speech and language construction; it’s simple and straightforward.

One last caveat: Esperanto is notoriously Western influenced, and is reportedly harder for Asian native speakers. I don’t believe a truly neutral language is possible, although Lojban is close: it’s equally hard for everyone, regardless of native tongue. (That’s a friendly dig at you Lojban folks, just a small joke).

Edit because it’s a nicely done book, some eye-candy:

I’ve been checking Esperanto now and then occasionally. Never got deep into it. Each conlang has its purpose, and Esperanto is a fascinating reflection of the era it was born from.

I speak some bits of Esperanto. Simple sentences, really; “mi nomiĝas [enmetu nomon]” tier. I could’ve progressed further on it if I wasn’t prone to the “oooh shiny” syndrome, and Sanskrit didn’t take so much of my time back then.

I also can read and write Tengwar, but I don’t know any conlang using it, not even Sindarin. (I use it mostly with Italian and Portuguese)

Beyond that I’m trying to learn the basics of my own conlangs Tarune and Meznagar as I’m making them. Specially the later as it has a bit more of a normal phonology, the former has stuff like pre/post-nasalised stops and uvulars that give me a hard time.

It’s been ages since I actually worked on any, but way back I got reasonably deep into Lojban

Another conlang I’ve been interested in for a long time is Toki Pona, and because it seems so easy to learn, I might start learning it too. The community for toki pona seems especially welcoming and wholesome, so that’s a plus too!

I don’t believe it will become the universal world language, like once was dreamed. And I do think esperanto has quite a few shortcomings when it comes to being an international auxilliary language.

Ohh, I’ve been thinking up a concept for a future of the world, and in this future Esperanto actually does end up an official language in two countries: in the European Federation, as one of the three federal lingue franche alongside Interslavic and European American Sign Language; and in the Union of the Leagues of Commons of Turtle Island as one of the two federal lingue franche alongside Plains Indian Sign Language. Which is to say that Esperanto is not used as a global language as such, but instead is used as a regional auxlang in places where speakers of different Romance and Germanic languages interact — and even then, the usage of Esperanto often involves simcom, ruby-like glosses, pictograms, and other aids to increase comprehension.

The actual global lingua franca in this timeline then ends up being, wouldn’t you know, International Sign! Which in the future, just as right now, is not a proper language, but rather a more spontaneous system of cross-language communication for users of different sign languages, which in this future the vast majority of hearing people have become.

So essentially, the technology and modes of production of the future have left the world with no real need or desire for a spoken global lingua franca, but rather there is a signed global lingua franca used by both Deaf and hearing people, and there are also a handful of spoken and signed regional lingue franche.

But anyways, yeah, I’m going to say Interslavic and Luka Pona, since OP already mentioned Esperanto and Toki Pona.

I think Duolingo has esperanto.