How do you express speaking a language that isn't a regular ajective? Do you just say "Я говорю по-иврит" or "по-эспэранто"; or do you add -ски anyway (по-ивритски, по-эспэрантоски) or anything else?
We use на + Prepositional (на иврите). If the name of the language is indeclinable, so be it (на эсперанто).
In Russian, saying someone "speaks" a language is not the only way to say that a person has some proficiency in it. We also use the verb знать (to know), which is, I think, more common.