Esperanto is a language constructed by Mr Zamenhof in the 19th century, and actually spoken by around two million people.
It also has its own Stack Exchange site already.
Should questions about Esperanto be on-topic here?
While questions about Esperanto wouldn't be off topic, they'd probably get a better answer on their existing dedicated site.
It's analogous to someone asking about a problem they were having developing a Wordpress plugin on Stack Overflow, when there is a site dedicated to that topic.
You'd get an answer, and probably a good one, on Stack Overflow, but the community on the Wordpress site are dedicated to answering just Wordpress questions and therefore should be able to give you a better answer.
So, I wouldn't explicitly ban questions about Esperanto, but I would point users at the dedicated site instead.
Just because another site for a topic exists doesn’t mean a question can’t still be on-topic here. The two sites are intended for two different but partially overlapping audiences. They will have two different general flavours and are likely to produce different sets of answers to the same question.
I imagine the Esperanto site will answer questions about Esperanto from the (spoken) Esperanto point of view: it works this way in this language because A and it doesn’t work that way because C and D.
I imagine this site will answer questions from a more conceptional or comparative direction: it was designed this way because of A and the similarity to X, while using C would have violated the principle D.
I suggest the following guideline:
If the question would be acceptable here after replacing all instances of and references to Esperanto with <insert random other conlang name here>, it should be considered on topic.
If not, it should be closed (but we can point towards the Esperanto site).