I have met some folks who cringe upon the mention of Pope Pius XII because they seem to be anti-Vatican I Old Catholics. They are heretics in their own way as many of them now accept ɧoɱosɛҳųαƖ unions, women priests, and have joined with Episcopalians. Not surprisingly, these Old Catholics often celebrate services in the vernacular, although you will find some that celebrate the Latin Mass. Without a Pope, they have become their own pope. They tend to dislike Pius XII especially because:

1) As a canon lawyer, the future Pope Pius XII worked on the Code of Canon Law of 1917, which changed canon law to be in conformance with Vatican I.

2) As Pope, Pius XII changed the Latin Missal several times by adding Saints to the Church Calendar and by changing the Services during Holy Week, notably the Good Friday Services.

3) As Pope, Pius XII allowed the Germans to celebrate the Mass in German during World War II. During that same war, the British also celebrated the Mass in the vernacular, notably the Jesuits, especially the heretic Teilhard de Chardin.

4) As Pope, Pius XII issued the Encyclical Mystici Corporus, which established a Diocesan Liturgical Commission in every diocese. These diocesan liturgical commission encouraged the use of the vernacular in the Mass, and ultimately brought on Vatican II.

and finally
5) Didn't Pope Pius XII shortly before his death start the work to bring on Vatican II to complete the work of Vatican I?

I've never heard this before.  Why did Pius XII allow Mass in the vernacular in Germany?  And did the British do this with permission, or without?  How widespread was it?

They are not implying that he was not a pope but not a TRADITIONAL pope. 

Sedevacantism is a term used to describe the position of those who hold, as a personal opinion, that a particular pope has crossed the line into formal heresy, in which case under divine and ecclesiastical law he is no longer pope. TRADITIO has not reached this opinion, because there are other theological positions that can deal with an errant pope

I do not know how widespread it was. However, I also heard that at Jesuit colleges here in the USA, the Mass was celebrated in the vernacular during the 1950s and early 1960s before it was even allowed. The Jesuits used the Latin missal English translations provided. This was done with permission of the various diocesan liturgical committees, especially those in San Francisco. Staff would produce mimeographed paper copies of the English Mass for those who did not have the Sunday Missals.

I was living in the San Francisco Bay Area during the late 1950s and early to late 1960s. Our parish priest was quite the liberal and had Jesuit friends. He introduced the English Mass at our parish in 1960 before it was even allowed by the U.S. Bishops. Several other parishes also introduced it. We followed our Sunday Missals and used the English translation provided for the Latin Mass. The priest read all the parts of the Mass in English, and no one batted an eyelash. We all thought it was legit and was part of an attempt to help evangelize the youth who did not know Latin.