"discriminated against the best-qualified candidates, just because they're not bilingual"

If they're not bilingual going into a diverse field requiring varied communication and aren't putting effort into rudimentary understanding of the countries official second language then they simply aren't the best candidate.

Duolingo is free, and putting that you're making effort on a resume would show that you're not an obnoxious prick who assumes every single person in the world should cater to their limitations.

Why would you assume some of the best qualified candidates who are not bilingual would not also include people who speak French but not English?

Why should they be required to have passble English skills when you are putting significantly less or no effort into again, an official language. Shouldn't they be able to focus on being the best expert they can be using the language their family taught them, and apply for a job based on who they are and not what society thinks they should be? Or should you both be making the tiniest effort to be better, just a bit every day and practice saying "salut" four times in a row having a complete and fulfilling francophone conversation.