Translation:One says that Esperanto is an easy language.
In English, that comma would be incorrect; also you optimize out "that" in that context (Strunk pushed for removing useless words). I guess the grammar is different in Esperanto.
In English, you'd have a sentence: "He says that Esperanto is easy." In this context, "He says Esepranto is easy" is equally correct and better form (it's more compact, thus more forceful: it stays more coherent in the listener's mind).
In Esperanto, the comma before "ke" is required, or simply convention? Is it correct to simply omit "ke"; and, if so, do you still need the comma? "Oni diras Esperanto estas facil lingvo" is correct or no?
The answer is that there are two different ideas which in English are both referred to by the word "that".
(1) "That thing over there." This that refers to an object or an idea which is physically or conceptually some small distance away. In Esperanto, this is "tio".
(2) "I want to tell you that there is a clear answer to your question." This that is being used to introduce a clause. In Esperanto, we use "ke" for this.
Esperanto oni is a non-specified / generic personal pronoun. In English it would in most cases probably be translated as a non-specific "one", "you", "they".
Considering you've recently leveled up to 7 in Esperanto, I guess you've already learned that the number "one" (i.e. 1) is called unu in Esperanto, but I'm adding this reply in case someone else has got the same question later on.
To distinguish between esperanto (one who hopes) and Esperanto (Esperanto - the language) the latter has an initial uppercase-"e".
The -ant- part is the present active participle (compare English's "~ing"; (explanation borrowed from Lernu! (lernu.net)), and esper'o means hope.
(Interesting fact: Originally, the language was called "Lingvo internacia" (or Internacia lingvo), i.e. "International language.)