Translating Craft form error message labels
You can't translate that field. I've tried it too once but it can't be done right now without effort because craft\elements\Userhas no attributeLabels() function in it in order to translate the attribute so all Yii2 does is to change newPassword to New Password but it doesn't translate anything.

You can either submit a feature request for it or do some hacky stuff.

I submitted a feature request in their git

Edit 13.08.2018 This commit changed it, you'll be able to translate those messages now