For the last sentence, “saisir” carry the meaning, without the familiar register:

Aussi doués qu’ils soient, il y a un aspect que les londoniens ne saisiront jamais. (As good as they may be, there’s an aspect Londoners will never truly succeed to understand/achieve)

The expression “avoir tout bon / avoir tout juste” is more familiar and is less linked with the meaning of “to understand”. However it sounds weird in the Londoners sentence, but you can use it for the meme: “les cupcakes-canards en plastique ont tout bon !”

However finding a perfect translation for a meme seems impossible, as meme English isn’t even correct English and full of double-subverted-meanings.

Considering the most up-voted definition available in the Urban Dictionary “you completed a task successfully or got something right . ie :You nailed it to the cross“, the closest translation would be réussi / nickel / parfait ! with “to nail” = réussir.

Aussi doués qu’ils puissent être, il y a quelque chose que les londoniens ne peuvent pas réussir.

To the extent that “nailed it” can mean “spot-on,” you could use “{être} En plein dans le mille” [here sarcastically] for the doughnut caption (from ‘Reverso’), but negating this for the Londoner example would be awkward, so for that you could consider “{être toujours} à côté de la plaque” (also from ‘Reverso’).

“Aussi doués qu’ils puissent être, il y a un aspect des Londoniens qui est toujours [un peu/légèrement] à côté de la plaque.”

You cannot really use the same word to translate “to nail” in both “As good as they may be, there’s an aspect Londoners cannot quite nail” and the picture, because they carrying different meaning.

In the sentence, it means “to understand something/to get the meaning of something right”, and corresponding slang would be “capter” or “piger”. Depending on the context, a colloquial translation would be:

Les Londoniens sont peut-être bons, mais il y a quelque chose qu’ils ne pigent/captent pas.

As for the picture caption, I agree with Papa Poule, I would use something like “(Bam !) Dans le mille !”. Here “to nail” has the meaning of “to manage to do something”.

I can’t think of a direct translation to “nailed it”, but “trop fort” might be a close substitute.
It also works in the negative, as in “les londoniens ne sont pas trop forts au ____”.
I’m not sure how common it may be in France, but in Quebec it’ll work fairly similarly.

Some further suggestions:

• il y a un aspect qui échappe toujours aux londoniens
• il y a un aspect qui reste hors de portée des londoniens
• il y a un aspect que les londoniens ne maîtrisent pas

As for “nailed it, “parfait” might work, or “parfaite maîtrise”.

If you’re looking for slang/real life oral French, a close approximation to “I nailed it” would be “J’ai tout déchiré”. It’s the kind of thing one would answer to a friend (not to a superior) when asked “How did your exam/job interview go?”.

For the meme specifically, I guess in French we’d rather use “Excellent”, “Parfait”, “Impeccable”, or variants like “Absolument parfait”.

As for the sentence, it quite depends on what the Londoners will never quite nail ; if you mean they don’t understand something:

Aussi doués qu’ils puissent être, il a y a quelque chose que les Londoniens ne capteront/pigeront jamais vraiment.

Or more idiomatic (and less correct):

Ils ont beau être doués, il y a un truc que les Londoniens ne pigeront jamais complètement.

On the other hand, if it’s something they can’t do correctly (cooking, let’s say…):

Aussi doués soient-ils, il y a quelque chose que les Londoniens n’arriveront jamais à faire correctement.

Hope it helps,

The answers I have seen so far are good, however “Nailed it” is slang, so I would suggest something of the same language level, such as:
“Trop bien géré”, which also includes the touch of irony.

I can also thing of another way, in the same manner as duck cake pic, saying ‘Nickel !’, like the metal, is a short way of expressing a spotless victory, something done without errors.

“Mettre le doigt sur quelque chose” could be used for “getting something right“.

Aussi doués qu’ils puissent être, il y a un aspect sur lequel les londoniens n’arrivent pas vraiment à mettre le doigt.

You also keep the figurative meaning of “pinning something somewhere“.

For what seems to be your main question, “cerner” (D. 2) comes very close in terms of figurative and literal meaning alike.

Aussi doués qu’ils peuvent être, il y a un aspect que les Londoniens n’arriveront jamais vraiment à cerner.

Concerning the sarcastic use, I would go for “ça c’est fait”, a well-known meme-ish locution which would be identically sarcastic in this context. The implied is “let’s quickly move on to the next thing”.

For what seems to be your main question, “cerner” (D. 2) comes very close in terms of figurative and literal meaning alike.

Aussi doués qu’ils peuvent être, il y a un aspect que les Londoniens n’arriveront jamais vraiment à cerner.

Concerning the sarcastic use, I would go for “ça c’est fait”, a well-known meme-ish locution which would be identically sarcastic in this context. The implied is “let’s quickly move on to the next thing”.

Quelques autres suggestions (bien que tardives) :
– “T’as assuré !”
– “Bien joué !”
– “Bingo !” / “Bravo !”
– “Mission accomplie !”
– “Et voilà le travail !”
– “10 sur 10 !”
– “C’est toujours un succès !” (Formule associée à un slogan publicitaire pour une marque de riz — points bonus si prononcé avec un accent créole en carton-pâte…)
– “Facile quand on a la technique !” (Provient également d’une vieille réclame mais j’ai oublié le contexte…)