A family of Microsoft word processing software products for creating web, email, and print documents.
I understand... But this is not the issue of your original message :-) It has nothing to do with AutoCorrect.
The suggestions in that list are supplied from the Speller Proofing Tool based on Word's recognition of the Language attribute applied to the text. It is context sensitive & the program will use the appropriate lexicon on the fly [if available], both for automatic spell checking & the main Spelling & Grammar check.
The Detect language automatically checkbox is a recent addition to Word 2016. This is a feature which Word for Mac never has had in prior releases & I'm not certain that the necessary underpinnings have yet been brought to a functional level. The feature works fine in the Windows versions as well as in Word Online, but your conclusion about its failure in Mac Office 2016 appears to be accurate. I am easily able to reproduce the issue here based on your example as well as others.
Please use the ☺︎ at the right end of the Word Ribbon to report your findings to Microsoft. You won't get a personal reply, but the more negative feedback they receive the higher the issue is placed on the list of priorities. Hopefully it will be resolved by an update in the near future.
In the meanwhile, all I can suggest is that you continue to use traditional methods: switch keyboard input for anything other than your default language and/or select text & directly apply the appropriate Language. Another alternative may be to work in Word Online.