Why's there no automatic keyboard language switch on iOS?

I like autocorrection and completion on my iPhone. It helps me type way faster with way less typing errors. I’m missing a feature that would probably improve typing even more. It happens quite often that I switch between the german and english keyboard on iOS. I message in german and write technical notes in english, my todos are a mix of boths. Writing a todo in german with a english word in it, such as function, results in an word correction by iOS. This is not what I want. I want to use english words within a german sentence. I do not want to switch to the english keyboard just for the word function and then switch back to german. Especially, when the keyboard switch is quite slow¹. There should be an option to prevent auto correction for words in the languages that you have activated as keyboards.

I would even further extend this and implement an automatic language switch when the os detects that I write in a different language. Similar how all these text programs are doing it. From a UX perspective I’d expect it like this:

1. Type english words while the german keyboard is activated
2. The autocorrection corrects my english words to german words
3. The os detects that I write in english after I typed a few words
4. The os automatically switches to the english keyboard
5. The os prompts me to revert its autocorrection for the last few words that I wrote. This way, I could ignore the autocorrection in the beginning and continue typing until the os detects that I write in a different language and automatically reverts its applied word correction.

If there are situations in which the switch is not useful, step 4. could be replaced as follows:

4. The os prompts me that it detected that I write in a different language and asks if I want to switch language and undo the autocorrection.

I believe this a quite reasonable and important UX improvement. Maybe somthing like this exists already with other keyboards from the App Store or on Android.