I discovered an interesting Welsh expression today – maen nhw’n datws o’r un rhych (‘they’re potatoes from the same furrow’), which is one equivalent of saying that they are as thick as thieves, i.e. they are close friends. Other Welsh equivalents of this expression include maen nhw’n gryn lawiau (‘they’re pretty (?) hands’); maen nhw’n yng nghegau ei gilydd (‘they’re in mouths together’); and maen nhw’n drwyn wrth drwyn (‘they’re nose to nose’).
In French the equivalent of this phrase is comme larrons en foire (‘like thieves in (a) fair’) – the word larron is a old word for thief – the usual word is voleur.
What about in other languages?
Two German expressions I can think of are sie stecken unter einer Decke ‘they lie under the same blanket’ and sie halten zusammen wie Pech und Schwefel ‘they stick together like pitch and brimstone’ — I think … wie Feuer und Schwefel ‘… like fire and brimstone’ can be used instead, but I’m not sure.
I was thinking two peas in a pod, but that is something different – meaning two people who are alike, not necessarily always together. We would also say joined at the hip.
The Dutch say onder één hoedje spelen ‘play under one hat’ (hoedje is diminutive, so literally it’s ‘small hat’).
Common Russian phrase for “joined at the hip” is не разлей-вода “cannot be separated by pouring water (on them)”, most probably alluding to pouring water on quarreling animals to separate them. And for being alike without necessarily being close there is одного поля ягоды “berries from the same field”, usually in negative sense.
Well… in colloquial Arabic dialects I can remember one similar phrase which is understood commonly between people of different background from west and east – however, it is a bit vulgar so I’m going to write the translation here only, which comes like: they are like two @$$es in one pant/underwear.
Well, as for the standard Arabic, I can’t remember something significant, but probably I’ve heard the expressions Two heads from the same shirt or Two bodies in the same shirt before.
In Finnish, “kuin paita ja peppu” (like a shirt and the butt).
A common neutral phrase in Spanish is “son uña y carne”, which means “they are like nail and meat”. But there’s a popular (or rather vulgar) phrase which is used, as far as I’ve read, especially in Catalonia and I don’t know if the original version is Spanish or Catalan. It’s “son culo y mierda” which means, if you’ll pardon the translation, “they are like ass and shit”.
In Portuguese it’s said “Farinha do mesmo saco” (Flour from the same sack) or “Unha e carne” (Nail and flesh).
I think that when you learn a language is really funny to look at the expressions and saying used, which sometimes can be really different from our mother tongue. If you require a Welsh translation make sure you address to professionals, to have the best results.