How to selectively enable autocorrect based on language?
There is no possibility to turn on autocorrect for one language on the keyboard but not on another. There is the setting "Check spelling that only underlines 'incorrect' words, but does not change it. I like to use that feature, as I am also bilingual. When that feature is on, you can first type your text and then tap the underlines one by one and then pick the suggestion you want. 

If you want to jailbreak your iPad, then you could install a tweak called 'ManualCorrect' that only changes the word to the suggestion when you tap it.

Source: own experience and http://iphonemonsta.com/manualcorrect-autocorrect-fix-iphone-ios-cydia