Jtdcftul

It is the middle of a moonless night when the people of Birmingham are awakened by a scream coming from the streets.
Someone has been murdered, they soon hear, but no body was found. Everyone gathers in the central square, looking for answers.

They all rightfully know that the Bishop would have been the only person awake and outside at the time of the events.
Disfigured by a disease that has affected thousands in his early childhood, he (like most of the few survivors) has never shown his face to anyone since then,
became a devout to thank God for sparing his life, and usually comes out only at night time.
When people come knocking at his door, he refuses to open as they would have expected. Instead, he speaks out the terrible truth that no one had been willing to see:
"Someone from Worcester came during the night. Count the children."

They soon confirm that one of the teenage boys is missing. A group of investigators is formed, and starts heading towards Worcester, accompanied by knights and diplomats.
In Worcester, they find that most of the city is out in the streets as well, with every child under the supervision of an adult. The Earl of Worcester advances towards them and speaks:

"One of my children has gone missing, and two knights including the one who was guarding him were found dead. Several of my people claim that a bishop from Birmingham is responsible for this crime.

-My Lord, we are terribly sorry to hear what happened here. Yet Birmingham has suffered the loss of a child as well, a teenage boy expert in French who cannot be found anywhere.

-Serves you right!" someone shouts from the crowd. "You stroke first!"

And as every face is turning to the corner where the voice has come from, a child turns up and hands out a piece of paper to the Earl of Worcester.
"Not everyone is who they claim they are", he says, and vanishes again into the crowd.

On the piece of paper, a chess board has been drawn:

What really happened during this dreadful night?

Hint #1:

Part of the answer requires to complete the French sentence by one word, in French.

See this lichess study for how the position is attainable

Here's the solution in story form:

The white knight was killed by the black pawn, which became a knight. The black knight was killed by the white pawn, which became a bishop. Another black knight killed the first white bishop.

So...

Birmingham=black and Worcester=white (for clarity, I will simply use "W" and "B". The light-squared bishop - that is, the trapped bishop on the c-file, hence the statement that he doesn't show his face and refuses to open the door - sees a visitor and knows someone is missing. Everyone assumes that since a boy is missing, a boy died. However, the knight died, and the boy became a knight.

They arrive in Worcester....

to find that W is on edge - everyone is guarding the children - and the town has apparently suffered three losses - a boy (the boy of whom it is said "he became" in French the hint hidden in the text, who became W's dark-squared Bishop), the knight guarding/behind that pawn dead, and another knight dead. Both of those knights actually died; however, the Bishop of W died and the missing child that killed the knight "became" the bishop of W.

"Serves you right" from a Worcest-ian indicates that

The two B knights went raiding in W's territory, along with a young boy, killing one of their bishops and two knights. They was pursued into B by one of W's boys/pawns, who kills one. W's boy/pawn earns the title of knight for his deeds, which the public are obviously aware of, thus the "Serves you right". B's boy/pawn that participated in the raid decides to try to replace the knight that died in carrying out the raid.

Also [IMPORTANT]

The French part is completed like so' "il est devenu un évêque" - "he became a bishop."

First, we have to notice

it never says the white pawn is dead.

From this, it's possible to deduce that

it's somehow still on the board.

which leads to

promotion, which also explains how the bishop on c1 got out!

However,

we have a problem. The white pawn has to promote on a dark square, which means it needs to promote on f8 or h8. However, the only piece that the white pawn to capture for it to promote there has to be the black g7-pawn. However, once it captures the pawn (after the g7-pawn captures the knight), it has to get past the f7 or h7-pawn, which I'm not sure how it can.

The white Pawn moved forward from his starting position onto a black square, when the Bishop, on his nightly wanders, captured him. In retaliation the white Knight captured the missing black Pawn but left himself open for the Bishop to also capture him and then return to his starting position.

In the meantime the rest of the white non-pawn pieces used the gap of the captured Knight and Pawn to move forward and protect the other Pawns.

I have an answer!

The one white pawn does have to kill the black pawn to get his promotion (there's nothing else to kill, and he needs to promote on a black space) but the pawn doesn't have to be a pawn at the time. It can go promote and then come back. That would let the white pawn capture into the black space.

Of course, there's a problem.

The white pawn and the black pawn have to somehow get past each other to make that happen, without disturbing the other pawns. At minimum, that requires two kills of the same color.

That's okay, though

White's apparently lost both knights - apparently a very recent edit.

So, final answer, without fratricide:

The white pawn and black pawn advance. Black pawn gets around white pawn by killing both knights, and promotes to queen. Black bishop moves out of the way, the newly promoted black pawn moves into his position, and is killed by the white pawn, promoting him to bishop, whereupon he comes back. Someone kills the old bishop, and white trundles the rest of his people out and into place.