A expressão "carta de chancelaria" pode ser traduzida para o inglês como "letter of credence." Neste contexto, uma "carta de chancelaria" é uma carta oficial assinada pelo Ministro das Relações Exteriores (ou equivalente) de um país de origem, que credencia um encarregado de negócios a.i. para exercer suas funções diplomáticas em outro país, como o Brasil.

An official document reporting a fact written by the Minister of Foreign Affairs is typically called a "diplomatic note" or "diplomatic dispatch." These documents are used in international relations and diplomacy to convey information, make formal requests, or communicate with foreign governments or diplomatic missions. The specific term used may vary by country and organization, but "diplomatic note" is a common and widely recognized term for such documents.

This article, from which the following paragraph was quoted, talks about the different types of presidential communication with foreign dignitaries, including "nota verbal" (notes verbales), "nota assinada" (signed note), "carta credencial" (letter of credentials), and "carta de chancelaria", as defined below:

Carta de chancelaria: forma de comunicação do presidente da República com o chefe de estado ou de governo, para informar decisões de estado, anunciar investidura, acreditar missões permanentes e especiais, revogar credenciais, responder a cartas credenciais e a outras comunicações de chefes de estado, manifestar pesar por falecimento, enviar felicitações, ou tratar de questões substantivas da agenda bilateral ou internacional. É redigida em forma solene e leva, além da assinatura do chefe de estado, também a assinatura do ministro das Relações Exteriores.

https://blog.grancursosonline.com.br/expedientes-comunicacao...

According to the aforementioned explanation, such letter is signed by both the head of state AND the minister of Foreign Affairs.

You may verify the translation of the keywords mentioned in paragraph 1 (above) on UN's Manual of Protocol:
https://www.un.org/dgacm/en/content/protocol/manual-of-proto...

Although there is no mention of "letter of chancery" or "chancery letter" in the Manual of Protocol, it does refer to:

(c) Household employees (if registered separately):

(i) An official letter (not a note verbale) addressed to the Chief of Protocol and signed by the Head of Chancery or the principal administrative officer, stating the name, occupational title and effective date of appointment;
https://www.un.org/dgacm/en/content/protocol/manual-of-proto...

There are plenty of references to "letter of chancery" or "chancery letter" on-line. I picked a couple for exemplification purposes.

Example Sentences

"Sold iwth named Buckingham Palace enclosure, in envelope; named Central Chancery letter..."
https://issuu.com/noonansauctions/docs/medals_13_sep_23_

A letter close, sealed by the small or privy seal, under John, is still a letter of chancery, which the chancellor
adopts, so to say, and enters with the letters, sealed with the " great " seal, on the roll of the year.
https://historyofeconomicthought.mcmaster.ca/tout/AdminHist0...

"I am directed to refer to letter of Chancery Embassy of Pakistan, Bishkek Kyrgyzstan undet
which he has forwatded the vedfication of your postgtaduate degree..." https://www.pmc.gov.pk/Documents/DCDecision/14-Misc-28(3).pd...