What we have is a letter written by Pacelli to Tisserant in response to a news article that had been sent to Tisserant reporting on legislation before the Polish parliament that would outlaw traditional halachic - schechita - slaughter of animals.
Firstly, let us place the text in its historical context.
1. Pacelli, acting in accord with Pope Pius XI, had regularly protested anti-Jewish and antisemitic legislation through the networks of nuncios and other papal representatives. The most well known are the dispatches Pacelli sent to the German nuncio, Archbishop Cesare Orsenigo ordering him to make formal protests to the German government over anti-Jewish persecution. Further, Pacelli also ordered the nuncio to do whatever he could to help German Jews. In reality, there was little more Pacelli could do; he relied on Orsenigo who demonstrated on more than a few occasions, a concerning lack of energy over Jewish matters.
2. This points to a series of obvious, but often overlooked, realities of Vatican diplomacy.
a) The pope and Pacelli could, and did, protest, but usually to little effect. Bound by the conventions of concordats with states such as Germany and Poland, which they observed to the letter, Vatican protests could only raise issues with the government. And the historical record is replete with examples of governments ignoring the Vatican. The irony in this case is that Poland's identity in the inter-war years was, in no small part, built on a publicly proclaimed Catholicism, albeit of a Polish rather than Roman slant. Neal Pease's book Rome's Most Faithful Daughter goes far to dispel many of the myths of Polish Catholicism.
b) The pope and Pacelli could, and did, stir the local bishops through the nuncio and direct communication, but that too was limited. The Vatican rarely ordered bishops to act on local matters outside of internal Church affairs, such as liturgy or appointments, preferring to trust that bishops would "do the right thing". Given the murky and long history of Judeaphobia, to expect Catholic bishops in central and eastern Europe to rally to the Jews was unlikely. It had, as historians such as Hubert Wolf, Emma Fattorini, Susan Zuccotti and Michael Phayer have pointed out, taken Rome decades in the modern era to come to a point where a condemnation of antisemitism was even feasible or desirable. Nazi persecution focused attention precisely because it was qualitatively different to previous manifestations.
c) Evidence from ADSS and a host of other sources, such as the Akten deutcher bischofe uber die lage der kirche 1933-1945, demonstrate that among the bishops of Germany and the rest of Europe, there was a wide diversity of opinion and attitude. The German bishops were often unable to agree on a united strategy to counter National Socialism, and during the war found themselves unable to agree on a plan of action to assist the Jews. The bishops of Poland generally accepted the teaching and effects of centuries of supercessionism and the theology of contempt towards Jews and Judaism, ensuring Jewish concerns would always be second to Catholic Poles. During the war this proved to be catastrophic; Jews would always be "lesser victims" to everyone else.
3. Poland 1938.
In the years following the death of Marshall Jozef Pilsudski (1867-1935), the Polish government had gradually introduced more restrictive anti-Jewish laws, building on a widespread social acceptance and approval that had long been a part of nationalist Polish self-identity. In June 1936, the prime minister, Felicjan Slawoj-Skladovski (1885-1962) declared his support for an economic war against the Jews. What had been an unofficial government policy that enjoyed considerable support from the Catholic clergy, now became official policy.
Removing Jews from Poland's economic life received vocal support from the Cardinal-Primate, August Hlond (1881-1948), archbishop of Poznan and Gniezno, who declared that Jews would always be a problem for Poland, always at odds with the Catholic Church and always at the beck and call of godless Bolshevism. A Poland without Jews would be a good thing.
The Sejm had already passed laws restricting and banning schechita - kosher slaughter - in 1923 and 1936. By 1938 the Sejm was also listening to calls not only for economic segregation but for physical segregation in the markets, housing, public transport, schools, universities, courts, and ultimately, forced emigration from Poland. Much of this enjoyed the public support of many Polish bishops.
When Pacelli wrote to Tisserant in May 1938, many of the Jews of Poland were facing economic ruin and poverty. They were, to use the title of Celia Stopnika Heller's book, On the Edge of Destruction.
4. What was known in Rome, May 1938.
Although the principal preoccupation in the Vatican in May 1938 was Hitler's visit to Rome, news from Poland flowed into the Holy See as it did from all the other papal representatives around the world. Pacelli's response demonstrates that the matter was noted and the nuncio to Poland informed. There was an attached report from Poland to the original letter, but it has not been reproduced on any of the sites I researched. I doubt the matter raised any "red flags" in Rome, indeed by comparison with other issues facing Polish Jews, kosher butcher restrictions were probably not considered of urgent importance. Indeed, it is of greater concern that the inflammatory statements made by more than a few of the Polish bishops appear to have gone unchallenged by the Vatican. I admit that the last sentence is not based on a comprehensive study of the matter, but I have not come across any disciplinary statement or request for caution from Rome.
From the Vatican 21 May 1938
Secretary of State of His Holiness
To be cited in response.
My Most Obedient, Eminent and Reverend Lord,
With the venerable letter of 6 April  which your Most Eminent Reverence was please to make known to me the article in the newspaper to the effect that the Government of Poland has planned to introduce a law which would prohibit the “slaughter by jugular”, imposed by Israelite religious law, and therefore constitute a real persecution of the Jews. Your letter insinuated, therefore, the convenience of a gesture of the Apostolic Nuncio to prevent this measure.
I did not fail to pass on this information to His Excellency, Archbishop Cortesi, and I am now happy to give your Eminence, in the attached copy of his Report Number 89 on 7 May , which contains precise information on the issue.
I gladly take the opportunity to express to your Eminence, the assurances of my profound veneration, and humbly kissing your hands,
I am your Reverend Eminence’s most humble, true and devoted servant,
E. Card. Pacelli
His Reverend Eminence, the Lord Cardinal Eugene Tisserant,
Secretary of the Sacred Congregation for the Eastern Church.
In 2008 Andrea Tornielli wrote an article on the document. It is interesting, but does not examine the historical context in sufficient detail. Further, Tornielli uses the document as evidence that Pacelli was not antisemitic. I disagree. The document is evidence that Cardinal Pacelli responded to a request from Cardinal Tisserant to help the Jews of Poland and protest at a proposed unjust law - which would add to a series of unjust laws that actively discriminated against Polish Jews. The question of whether Pacelli was antisemitic or not is not applicable here.