Spell check multiple languages at the same time, at the same input field?
The best you can do for apps that use the Apple facilities is to go to system preferences/keyboard/text/spelling/setup (at the bottom of the list) and check the boxes for only those languages you want to check, and then set it to "automatic".

How well it works for your particular requirements can only be determined by testing.  As far as I know, multilingual only works with languages for which dictionaries are provided by Apple.

Some apps like MS Word have their own spellcheck systems.