Holy faux amis, Batman! Beware of sound-alike cognates!

I don't speak French, but I'm pretty sure that c'est formidable more or less means "It's amazing!" or "It's excellent!", but usually NOT "It's formidable!" Because in English, the word "formidable" suggests a THREAT, and is a synonym for "intimidating" or "just a bit scary."

Admittedly, there are contexts in which English "formidable" has both meanings ("excellent and threatening"), as in "He is a formidable chess player." But you can't say "This chocolate cake is formidable!" in the sense of "amazingly excellent". You can, however, say "The recipe for making this chocolate cake was formidable" (meaning "the recipe was so complicated, I was afraid to attempt it").

Incidentally, Google Translate renders English "formidable" as грозный, but the phrase "it's formidable" is rendered as это превосходно. The first translation definitely seems correct, but I have my doubts about "превосходный".

But in a French-to-Russian translation, Google gives это здорово! for "c'est formidable" -- which, I think, is also correct, if you understand "Здорово!" to mean "Amazing! Excellent! Awesome!"

P.S. When Poirot said "a formidable woman," he meant it in the same sense as "a chess expert I would be afraid to play against."