I am translating a British story and I came across this expression
"you look like you lost a shilling and found a penny". I am not sure I understand what it means.
A shilling and a penny are two increments of currency, where a shilling is made up of many pence (the multiple for penny when it is a unit of currency). In Britain, actually, the shilling is no longer in use, so this is somewhat of an archaic saying.
Essentially, though, the saying implies that you've lost something, and found something less valuable to replace it — or even, simply that you look disappointed about something.
Prior to 'decimalisation' in February 1971, British currency followed the structure that had been used since pre-800 AD. This was as follows:
On decimalisation in 1971, the value of the pound was retained unchanged, but became subdivided into 100 new pence (abbreviation p) (the new has since been dropped).
The name shilling was dropped, but the then one shilling coin became worth 5 new pence. All pre-decimalisation coins of value sixpence (half shilling) or higher simply adopted their corresponding new values, and pre-decimalisation coins of lower value (which had no exact post-decimalisation equivalent) were withdraw and replaced by 'copper' coins of value ½p (since dropped), 1p & 2p (still in use, although coin size has changed).
So back to the original question:
refers to the pre-decimalisation one-shilling and one-penny coins, and (as others have intimated) was an old British idiom indicating that you had lost something of value and found something of considerably lower value.
The pre-decimalisation shilling coins remained in use until 1991, but their new equivalent, a smaller five-(new)pence coin, continues in use. Sadly, the old one-penny coin is long deceased.
Finally, clarification of plural of penny (supplementing the answer from @Hannele):