In macOS there appear words that are underlined in red if the spelling is wrong. However, I started to learn French and only found that French words that are spelled correctly have he red underlines because there are no such words in English.
Is it possible not to apply the spelling check on French words? I added French on the Language and Region setting on System Preferences but still have the underlines…
UPDATE
Here is the result of adding Français to the list.
But the preferences on text at keyboard looks like the following:
The relevant item is in the middle and it reads as Based on languages spell automatically.
And French words still have the underlines.
This is on Note.app and on macOS 10.14.3.
Best Answer
Language and Region doesn't really have anything to do with spelling in MacOS. Instead go to System Preferences/Keyboard/Text/Spelling and set the language there, plus maybe in Edit > Spelling and Grammar > Show Spelling and Grammar if necessary.
Some non-Apple apps like Word have their own independent spelling settings.
If you need to do both languages at once, you can try the Automatic setting in Spelling, after going to the Setup item and checking the boxes for just those two languages. How well this works in practice you will have to determine by testing with your particular text.
A better solution would be to use a more advanced app like Pages or Word, where you should be able to set the language of individual text runs so that the right spell check is used for each.
I think it may work better in iOS. If you have such a device, activate the French and English keyboards and dictionaries. My initial test indicates it handles a text like yours correctly.