I spend most on my coding and writing time on Atom and as I write in both English and French I use two dictionaries with spell-check. Well, it actually took me a while to understand how to use two different locales on my Debian machine simply because I was lazy enough to not read the README on the Github repository of spell-check. Out of curiosity, I checked out what hunspell is (note that the project is hosted on Github) and then I checked the dictionaries currently installed on my device:

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$ ls /usr/share/hunspell
en_US.aff en_US.dic

So, in order to use French dictionaries, I installed the following package:

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$ sudo apt-get install hunspell-fr-modern/testing

Then I re-check the dictionaries installed:

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$ ls /usr/share/hunspell
en_US.aff  fr.aff     fr_BE.dic  fr_CA.dic  fr_CH.dic  fr_FR.aff  fr_LU.aff  fr_MC.aff
en_US.dic  fr_BE.aff  fr_CA.aff  fr_CH.aff  fr.dic     fr_FR.dic  fr_LU.dic  fr_MC.dic

I was then able to use then in spell-check

Everything works just fine now! Note that hunspell is also used by Firefox and Thunderbird 😏!