とある映画を文庫化した—いや、映画の為に書かれたシナリオを小説として加筆修正し、日本語にローカライズしたものだ。

—、、。

It's a novelization of a certain movie — no, a novel adaptation of the script written for the movie, rewritten, expanded, and localized into Japanese.

Literal

A-certain movie [obj-を] paperback-made-[past-した]—no, movie [for-の-ために] written scenario [obj-を] novel [as-として] addition-correction-did, Japanese [to-に] localized [nominalizer-もの] is.

A self-correction in action. とある is a literary 'a certain' (more specific than ある alone). 文庫化 ('paperback-ization') uses the productive suffix ~化 ('-ization, becoming'). The em dash + いや ('no') signals the speaker reaching for a more precise formulation. ~として ('as, in the role of') marks the capacity in which something is regarded. 加筆修正 is a four-character compound: 加筆 ('adding writing') + 修正 ('correction'), meaning 'revision with additions.' ローカライズ is the direct loan 'localize.' A technical description showing Japanese's flexibility in discussing publishing.