"English is a language."
Translation:La angla estas lingvo.
"lingvo", after the verb "to be", or "to become", which refers back to the subject is a predicate nominative and not a direct object and so will not be in the accusative case.
"English is a language." English = language. English (subject) is (linking verb) a (indefinite article required in English, but does not exist in Esperanto) language ( predicate nominative, also called predicate complement)
I speak the language. "mi parolas la lingvon." Here "lingvon" is not the same thing as the subject "mi". I speak what? the language ( "lingvon" is the direct object of the verb "speak" and so it is in the accusative case and takes the -n )