"People throughout the country speak Esperanto."
Translation:Homoj tra la lando parolas Esperanton.
Most languages are referred to with an adjective -- e.g. la angla estas bela lingvo; mi parolas la anglan. The noun lingvo is usually omitted, though la angla lingvo estas bela lingvo; mi parolas la anglan lingvon would also be possible. Much as we might say "I can speak the English language" in English.
But a few languages have names of their own that are nouns, rather than adjectives modifying lingvo; these include Esperanto.
Sometimes I revert to what I first learned when naming a language being spoken and give it an -e ending (Mi parolas … Esperante, angle, ĉine, jorube "I am speaking in Esperanto… ktp) and, as far as I can determine that form is still acceptable and understandable in Esperanto. But Duo doesn't like it.
In that form, it is an adverb modifying the verb "speak".
A possible albeit clumsy translation in English would be, "I speak Esperantishly, Englishly, Chinese-ishly, and Yorubishly."
In English, a phrase like this doesn't indicate that you speak such language, but that you speak in a manner that a speaker of such language would speak.
I don't know if this is the case in Esperanto, but perhaps the adverbial form has a different meaning.