"People throughout the country speak Esperanto."
Translation:Homoj tra la lando parolas Esperanton.
My dictionary says that tra = through, rekte tra = throughout. This question only accepts tra.
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.
Those few languages are either "dead" languages [Latina, Babilonia, Antikva Helena] or conlangs, [Esperanta, Idoa, Volapuka]
There may be exemptions, but I haven't seen any yet.
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.