Spell check multiple languages at the same time, at the same input field? Is there an option to somehow combine multiple languages at the same spell check "session"? Form what I can find under Keyboard > Text it seems that there are only two options
1) Chose one specific language
2) Allow to detect language by text that is written

I do often mix words from multiple languages and it is very annoying that I have text that has red line under words. I do not want to turn off spellcheck entirely. I still need to verify that what I have written is correct. I just need to combo my preferred languages.
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.