You hardly need to look at Mediator Dei (which, if it's a blueprint for reform at all, is a very minor reform)..
Hey why don't we look at
Mediator Dei together? Here is paragraph 60:
"60. The use of the Latin language, customary in a considerable portion of the Church, is a manifest and beautiful sign of unity, as well as an effective antidote for any corruption of doctrinal truth.
In spite of this, the use of the mother tongue in connection with several of the rites may be of much advantage to the people. But the Apostolic See alone is empowered to grant this permission. It is forbidden, therefore, to take any action whatever of this nature without having requested and obtained such consent, since the sacred liturgy, as We have said, is entirely subject to the discretion and approval of the Holy See."
Italics mine. Now I know that I have stumbled whenever on a Tradland website whenever I see the first sentence quoted dotingly and the rest of the paragraph omitted. It should be noted that Thomists have a tendency to give the argument for the rejected position first.
Here is a more obscure bit of trivia about something Pius did back in 1939 that suggests he saw value in vernacularization:
http://shanghaiscrap.com/2007/07/in-the-vernacular/ He appointed Bugnini to the "Commission for Liturgical Reform" not the "Commission for Liturgical Ossification"
Pope Pius was an admirer of Jungmann and was in fact leaning towards vernacularization as well as other changes. However he was an incrementalist who wanted to phase the reforms in gradually and not at the torrid pace of OTL 1960's. He also insisted on centralized control of the process which broke down in OTL. Just before his death he approved the "Dialogue Mass" which is in accord with para 105 of
Mediator Dei. His health problems may have slowed down the pace of reforms. Another complication in the process was his convoluted up and down relationship with Cardinal Spellman who was a strong opponent of vernacularization.