原文 and 訳文: the source text and translated result
Identify source and translated texts by their relationship, without fixing their languages or treating comparison as proof of agreement.
The text used as the source
「翻訳と原文を一行ずつ比べた。」 describes comparing a translation with its original line by line. 原文 names the source used in that translation relationship; the sentence does not identify its language.
「引用した部分が原文どおりか確認した。」 describes checking whether a quoted section matched the original. The quoted section is the object of the check, but the sentence does not state the outcome of that comparison.
The text produced in translation
「原文と訳文を一行ずつ比べた。」 compares the source with the text translated from it. 訳文 names the translated result, rather than a text that must always be in English or Korean.
「読みやすい訳文になるよう語順を整えた。」 describes refining word order to make the translation easy to read. Retain that action and goal without adding that the accuracy of every detail was verified.
Name the roles by their relationship
原文 and 訳文 identify the starting material and translated result. A text written in Japanese is not automatically the 原文 in every translation relationship.
The examples do not specify source or target languages, so do not supply a particular language direction as a fact. The focus is which text supplies the source and which is its translation, rather than a choice of literal or free translation.
What roles do 原文 and 訳文 have in 「原文と訳文を一行ずつ比べた。」, and does the sentence establish that Japanese was the source language?
Show the answer and why
原文 is the source text and 訳文 is the translated result. Neither language is specified, so the sentence does not establish Japanese as the source language. Comparing the texts line by line also does not itself prove that all their meanings agreed.