In Dutch you know something (or someone!) like your (trouser) pocket (iets of iemand kennen als je broekzak), so roughly the same as in French.
I suspect it started with knowing an area like the inside of your pocket (as in knowing your way around), but it’s used for knowing people very well now, too.

The Spanish would be “Conozco ese como la palma de mi mano” (I know it like the palm of my hand), but thank you for the idiom nonetheless!

In Hungarian, „Úgy ismerem, mint a tenyeremet” = I know it like my palm(s).

In Catalan, if it is for a person, you can say:
El conec com si l’hagués parit (slang)
which means:
I know him as if I had given birth to him

“Conozco ese…” makes no sense in Spanish in that phrase. It would be like saying in English: ‘I it know’ instead of ‘I know it’.
I’m a native speaker of Spanish since I am from Argentina. For example, if you’re talking about a girl or a city, which is femenine is Spanish, you can say: “La conozco como la palma de mi mano.” or if you’re talking about a boy or a town, which is masculine in Spanish, you can say: “Lo conozco como la palma de mi mano.”

“Conozco ese” can be used in a very different situation, say, you’re shown two CDs by a same band, and you say “conozco ese” meaning: I know about CD 1 but I haven’t heard of CD 2.

Excellent blog!

In standard Arabic I think the idiom is the same, with the palm of the hand. But in the Kuwaiti dialect there is a more common expression that goes like “I know its tummy and back”, or something like it.

In Russian, я это знаю, как свои пять пальцев, “I know it like my five fingers.”

In Chinese it’s 了如指掌 “know like fingers palm”
The first character is sometimes substituted for other characters that basically mean the same thing (know, understand, etc.).

On the other hand, if you’ve learned something by heart,
you say you’ve learned it till 滾瓜爛熟 “boil gourd soft cooked”.

Funny how people separately and independently come up with such similar ways of describing things. We’re more similar than we think.

Cheers,
Andrew

You already mentioned the sentence in Spanish. However, there is also a funny expression involving a part of the body. When someone says that two people are alike, and I think instead that they are very different, I can answer him saying “Se parecen en el blanco de los ojos” (so only the white portions of their eyes look alike) :-)

Czech: “znám to (ho, ji) jako své boty” = “I know it (him, her) like my own shoes”.

In Swedish if you have learned something really well, you can say that you know it like a flowing waterfall.

In Finnish, “Tunnen sen kuin omat taskuni” (I know it like my own pockets). I don’t think it would be used of people.

- In Tamil, there’s the following expression: “அவர் வேதத்தையே கரைத்துக் குடித்தவர்..!” (avar vEdattaiyE karaittu-k-kuDittavar) = lit: ‘He’s dissolved/melted down the vEda-s [ancient Sanskrit scriptures) and drunk them’

–> i.e. ‘He knows the vEda-s back to front/inside out..!’. Can’t use the same phrase for people, though.

And then there is the english comment to sarcastically suggest the opposite…He/she knows it like the back of his/her head.

In Lithuanian “Pažįstu kaip savo penkis pirštus.” (I know it like my five fingers.)

In Portuguese is very similar to spanish (very often this happens), and we say Eu o conheço como a palma da minha mão formally and unformally something close to “conheço ele com a palma da minha mão” (well, at least in Brazil : ))

In Russian – ” Snatj kak swoi pjatj palzew”(to know it like your five fingers)

In Polish:
Znać coś jak własną kieszeń (to know sth like their own pocket) – to know very well
znać się jak łyse konie (to know each other like bold horses, which has meaning: to know each other very very well:)

In European Portuguese (as spoken in Portugal), the idiom is much the same as in Spanish. «Conheço-o/-a como a palma da minha mão» (for people) («I know him/her like the palm of my hand.») Also: «Conheço este lugar como a palma da minha mão.» (for places) («I know this place like the palm of my hand.»)