When you refer to a language, you must have la in front of it, because it is short for la ... lingvo.
For example, English is la angla, short for la angla lingvo.
A few languages, for example Esperanto, that don't belong to a certain people or a certain place, have their own names ending in -o (without la). So you would say, Mi parolas Esperanton and not Mi parolas la Esperanton.
Oni is the indefinite third person pronoun, equivalent to "one" in English. It is used frequently in Esperanto, more often than the pronoun "one" is used in English. One can use it both for general statements, and to use the active instead of the passive voice:
Oni diras, ke la angla estas malfacila lingvo.
People say that English is a difficult language.
Oni parolas Esperanton en la domo.
Esperanto is spoken in the house.
Subordinate clauses are often introduced by ke ("that"): Li diras, ke vi parolas Esperanton.In English, It is possible to leave out "that", and say "He says you speak Esperanto", instead of "He says that you speak Esperanto." However, in Esperanto ke must always be included.
Note also that there is always a comma before ke, though the English translation may not have one.