In French, to address a letter to whom it may concern I would use:
Inside the text of the letter, you could use the à qui de droit translation:
You should forward this letter to whom it may concern
Vous pouvez transférer cette lettre à qui de droit
In French, it’s not correct to adress a letter à qui de droit (“to whom it may concern”).
It is preferable tu use Madame, Monsieur, (“Dear Sir or Madam”) if there is one person or Mesdames, Messieurs, (“Ladies and Gentlemen,” but this translation is rather awkward) if there are several people, followed by a new paragraph.
On top of other answers,
Another translation of to whom it may concern is aux intéressés:
Merci de communiquer ces informations aux intéressés. (Please communicate this information to whom it may concern)
Should you read this a day, the meaning is basically the same as à qui de droit. The slight differences between the two are mostly a question of usage.
For example, a legal request (permit, authorization…) needs approval from qui de droit while information can be communicated aux intéressés or à qui de droit.
Aux personnes concernées is sometimes used as well but is less common. The meaning is the same as aux intéressés, in a lower register:
Merci de communiquer ces informations aux personnes concernées. (Please communicate this information to whom it may concern)
Stumble into this thread as I couldn’t figure out “To whom it may concern” in my native language when used as a starter for a non-personal mail (say, to an administration). It eventually struck me:
À l’attention des personnes concernées