YOU SAID:

If a translation engine were perfect, then translating a phrase from (say) English to Japanese and back again would leave us with the phrase we started with.

INTO JAPANESE

翻訳エンジンが完璧であれば、フレーズを（たとえば）英語から日本語に翻訳し、再度日本語に翻訳すると、最初のフレーズが残るでしょう。

BACK INTO ENGLISH

If the translation engine were perfect, you could translate a phrase from (for example) English to Japanese and then translate it back to Japanese and the first phrase would remain.

INTO JAPANESE

翻訳エンジンが完璧であれば、（たとえば）英語から日本語にフレーズを翻訳し、それから日本語に戻すと、最初のフレーズが残るでしょう。

BACK INTO ENGLISH

If the translation engine were perfect, (for example) translating a phrase from English to Japanese and then back to Japanese would leave the first phrase.

INTO JAPANESE

翻訳エンジンが完璧であれば、(たとえば) フレーズを英語から日本語に翻訳し、その後日本語に戻すと、最初のフレーズが残ります。

BACK INTO ENGLISH

If the translation engine were perfect, you could (say) translate a phrase from English to Japanese and then back to Japanese and you would be left with the first phrase.

INTO JAPANESE

翻訳エンジンが完璧であれば、フレーズを英語から日本語に翻訳し、その後日本語に戻すと、最初のフレーズが残ることになります。

BACK INTO ENGLISH

If the translation engine were perfect, you would translate a phrase from English to Japanese, then back to Japanese, and you would be left with the first phrase.

INTO JAPANESE

翻訳エンジンが完璧であれば、フレーズを英語から日本語に翻訳し、その後日本語に戻すと、最初のフレーズが残ることになります。

BACK INTO ENGLISH

If the translation engine were perfect, you would translate a phrase from English to Japanese, then back to Japanese, and you would be left with the first phrase.