What’s the best translation of the intensifier “the fuck” in other languages?

Modern Greek.

“What” (τι) questions will have σκατά “shit” inserted after it: τι κοιτάζεις “what are you looking at” > τι σκατά κοιτάζεις “what shit are you looking at”.

The more generic intensifiers are στο διάολο “to the devil”, for interrogative sentences, or ρε γαμώτο “for fuck’s sake; literally hey, I fuck it”, for other sentences.

Τι κοιτάζεις > τι στο διάολο κοιτάζεις “what the hell are you looking at?

Πότε στο διάολο θα έρθει “when the hell will he get here?”

Κοιμήσου ρε γαμώτο “Sleep, for fuck’s sake”.

Ρε γαμώτο, νόστιμο είναι το φαΐ “Fuck, that food is tasty”

Γαμώτο has the historical linguistic distinction of being an ossified piece of mediaeval Greek, in which the “it” follows rather than precedes the verb (γαμώ το); in Modern greek, “I fuck it” would be το γαμώ. Because it is an ossified piece of mediaeval Greek,

• people treat it as an exclamation: Voula Patoulidou, Olympic gold medalist at the 100m hurdles, exclaimed Για την Ελλάδα ρε γαμώτο θα τρέξω, και για κανέναν άλλο! “I’m running for fucking Greece, and nobody else!” And her patriotism was widely held to excuse her profanity. Για την Ελλάδα ρε γαμώτο “For fucking Greece” was a catchphrase for several years afterwards.
• people also treat it as a noun: the expression το γαμώτο της υπόθεσης “the ‘fuck it!’ of the issue is…” corresponds to “the catch is…, the rub is…”: it’s the part of an issue that makes you exclaim “for fuck’s sake!”