勘弁願いたい

「ええ。ウチとしても、精霊関係で人的被害が出るのは勘弁願いたいのよ」

Hi. Why is this sentence translated as "Yeah. We wouldn't want casualties to result due to the Spirits."? Where is "wouldn't"? I have looked up the expression 勘弁願いたい and found that it means "please forgive", not "wouldn't".
Thank you.

This sounds like a case of subjunctive mood (仮定法) to me. In English we distinguish these two:

(a) I don't want it to happen

(b) I wouldn't want it to happen.

The difference here is the use and non-use of subjunctive mood. It is my understanding that Japanese does not use subjunctive mood to distinguish (a) and (b) the way English does.

The best dictionary entry I see on this is (3): spare


which may not be entirely obvious how it's used. But in any case, ～勘弁+request is often used to mean 'please spare me from ~',

e.g.,

「私の身の上話をしてあげようか」「いや! それだけは勘弁してくれ! 《聞きたくない》」
"Shall I tell you the story of my life?"—"No! Spare me that!"


So, the grammar is not structured as a negative, but a request/desire for 勘弁 of an action is a request/desire for that action to not happen.

Thank you, Chris.