George Orwell wrote that mixed metaphors are a sure sign of the corruption of discourse, because if they are mixed, they must be dead. They refer to what is unspeakable. The language of contemporary politics is worse, more sinister, than blather, in just the way he meant; but language is full of dead metaphors, such as the seafaring metaphors of English, that are almost its semantic undercurrent. Go back further to the fossilised strata revealed by etymology, and the death of metaphor starts to look like the principal material of language. When poets conjure its ghosts — as Shakespeare did with sailors’ talk — a sea change can occur.
“Congenial” is about as untranslatable as sympathique, which is to say it’s not much trouble in practice, but should you wish to make sense of the sort of things it can be applied to — to find one rendering in another tongue that fits all its cases, and thus carries something like the same feeling, whatever it has in it that is more than just “nice” — well, it doesn’t seem possible. Sympathetic people and places — in the Gallic sense — carry shared feelings, that is, human commonalities. “Congenial” looks like it was coined to suggest a shared character, ingenium. But there is a specific sense from the language of plant breeding: congenial species can be crossed, or grafted (as hemp on cannabis). They take. ingenium is still there underneath the word, but might now be translated as “nature”. But never mind that; the notion of fruitful compatibility usefully abstracts from shared feelings. It makes quite as much sense to use it of a country or place as of a person, whereas for sympa and its cognates, that is already a stretch.
The language of nineteenth-century science has other such survivals, for instance (and especially in Germany, because of Goethe’s novel) “elective affinities” — which (if I remember O-level chemistry right) we know as ionic bonds. It’s that reaction where the compounds in two solutions swap partners when mixed.