Learn French: « Cette réplique est plus longue que l'autre », réplique-t-il. - "This line is longer than the other," he retorts.

This sentence uses a reporting clause (an incise) to show who is speaking. In written French, these clauses always invert the subject and verb. Because the verb réplique ends in a vowel and the pronoun il starts with one, the -t- is inserted to prevent two vowel sounds from clashing.

Is it strictly necessary to invert the subject and verb after a quote?

In written French, yes. When the reporting clause comes after or in the middle of a direct quote, you must invert the subject and verb (réplique-t-il). You cannot say il réplique in this position, even though in English we normally say "he retorts" rather than "retorts he."

Why is the word réplique used twice but translated as "line" and then "retorts"?

It is a deliberate play on words! The first réplique is a noun that means a "line" of dialogue in a play or movie, or a "retort". The second réplique is the conjugated verb répliquer, which means "to reply" or "to retort".

Why is the adjective spelled longue instead of long?

It is describing the noun réplique, which is feminine. The masculine adjective long has an irregular feminine form: we must add a "u" before the "e" to keep the hard "g" sound, making it longue.