In Firefox you can only select one dictionary at the same time AFAIK, so I'm afraid that what you want is not possible. You can search the Add-ons website to see whether anything suitable comes up.

Looking forward for this in a native manner.

Another bit of information: I'm using Firefox 57 beta, most of the addons is broken.

Re your wish for a multiple language spell checker :

The only add-on I could find is the Dictionary Switcher, but : a) it doesn't look like it will work with Firefox 57+, as the developer hasn't updated his add-on in years, b) the reviews aren't all that good.

https://addons.mozilla.org/en-US/firefox/addon/dictionary-switcher/

But you could try and contact the developer .......

Re your broken add-ons, would you please take a look at this :

https://blog.mozilla.org/addons/2017/09/28/webextensions-in-firefox-57/