Pius XI, Mit Brennender Sorge (publisher's location: publisher, date), chapter number.

If used more than once, the next time can be stylised as:

Pius XI, Mit Brennender Sorge, 3

if used immediately after the same source, like this:

Ibid. 3

In the bibliography, like this:

Pius XI, Mit Brennender Sorge. publisher's location: publisher, date.

If using a translation of the text,

Bloggs, Joe trans. Mit Brennender Sorge. (publisher's location: publisher, date), chapter number.

OR:

Pius XI, Mit Brennender Sorge trans. Joe Bloggs (publisher's location: publisher, date) chapter number.

Here is the full text in English, direct from the Vatican as the official translation. It's also available in its original German as well as Italian and Spanish.

Just a note: an oddity with this encyclical is that it is one of three issued in March of 1937. Three encyclicals in one month is incredibly rare. It came out on the 14th, Divini Redemptoris came out five days later on the 19th, and Nos Es Muy Conocida came out on the 28th. The contrast in how these three encyclicals were received by various populations can be very enlightening if one were interested in researching them.