How to manually set the language for Auto Correct? How can I manually select the language of Auto Correct for a block of text in apps like Notes and Mail?

I want to write a text that contains both English and German sentences. I have found a solution to modifying the setting globally in this thread, but I would like to have a mixed document.

If I type an English and German sentences, the resulting spelling corrections may or may not be reasonable and the pattern (if one exists) is not obvious. To be specific, in the following snapshot the second sentence is in the wrong language while the first and the third one are correct:



How can I change the spelling language of the second sentence to English?
You can try selecting the text and then setting the correct dictionary in Edit > Spelling and Grammar > Show Spelling and Grammar.  

It's not possible to actually tag text as being in a certain language in such apps.  You are at the mercy of whatever algorithm Apple is using to implement its concept of "automatic by language", which I think few users find very useful.