I learned Esperanto as a stepping stone to other European languages. Assuming I already know English and Esperanto (1 crown on all Duolingo lessons), which natural language(s) would be easiest to learn?
In terms of vocabulary I think Esperanto borrows heavily from French. I personally found my knowledge of Esperanto very useful when I started learning French. However I think that any romance language (Spanish, Italian, Catalan etc) will have most of this advantage, if to a lesser degree.
When people say that Esperanto is useful as a gateway language, I think this is mostly true in the sense that the most difficult part of learning one’s first language as an adult is learning how to learn, ie, what methods work best for you, getting a habit of practicing daily and building up the confidence to not worry about sounding like an idiot when talking to people. Esperanto is good as a first language to use for this because it is more regular so you can more quickly get to a point where you can express yourself. Also almost everyone who speaks it went through the same process so people are more sympathetic to making mistakes and there is less of a notion of the “right” way to say something like there is in national languages. In that regard having some experience learning Esperanto will help with any language. This is the route I took and from my personal experience as a monolingual Brit I can say I never would have had the confidence to try another language if it hadn’t have been for my positive experience with Esperanto.
I'm assuming your first language is Chinese, so that is another variable you want to consider before deciding which language would be easier to learn next. That is why I want to recommend Spanish to you, which I think is an easier Western language to learn. Phonetically, it is more like Esperanto. True, Esperanto has more French words, but from the perspective of Writing/Pronunciation, Spanish is much easier to learn and the grammar is not quite as complex. The only that is harder is the number of conjugation forms to learn.
I tried Romanian via Duolingo & was very easy. However, I speak (not write) Vlach(Aromanian, less Slavic elements, but I speak/write YugoSlav languages as well). It improved my overall understanding & grammar of other languages. Except that I apply Esperanto logic to English & now make (intentionally, no A/AN etc) more mistakes in English (LOL).