1.  Latin was the language of the mass.  Was any part of the service
other than the homily conducted in the vernacular?
2.  In what language were the scriptural readings given?
3.  What might the scriptural readings have been for Assumption Day? 
(Speculation is acceptable here.)

Time period is the first decade of the 14th century.  Place of interest is France.

Thank you,
Archae0pteryx

Greetings Tryx,
What a pleasure to see you here again, and one of your interesting questions!

Perhaps these sites can add some light:

http://www.fordham.edu/halsall/basis/latinmass.html
http://www.fordham.edu/halsall/source/busch-vernac.html

This one is especially interesting as it gives a first hand impression
of the situation a bit later in nearby Holland.  What stands out is
the clerics' distrust of vernacular religious texts, and also a
statement that a cleric "often preached" in the vernacular, suggesting
that this was not always the case for that man, and also worthy of
general remark.  From this, I would assume that vernacular readings of
the lessons would not have been usual, since the Church did not
approve the texts, although it seems that things were less strict
prior to the Reformation and Counter Reformation.

http://mb-soft.com/believe/txo/mass.htm

This leads to this excerpt from another site:
"The Council of Trent was convened by Pope Paul III in 1545 and
continued until 1563 under the leadership of four different popes
during a time characterized by political turmoil. It established the
system of seminaries to improve the quality of clergy formation and
decreed that each bishop had to reside in his own diocese. In response
to the Protestant Reformers, this council affirmed the Catholic
Church's traditional beliefs in the sacrificial nature of the
Eucharist and in the doctrine of the Real Presence. It also called for
the continued used of Latin in liturgy, although there was no specific
condemnation of the use of vernacular. In fact, the council fathers
decreed that vernacular explanations of some of the liturgical texts
had to be given in the context of liturgy on every Sunday and holy
day.[1]"
http://www.liturgica.com/html/litWLTrent.jsp?hostname=null

The last sentence suggests strongly that the lessons were read in
Latin, since only they would be a part of liturgy that varied in
content and could require explanation.  Of course, this is 200 years
later than your period, but when one recalls that traditionally the
sermon or homily was an exegesis of the verse or lesson for the day, I
would expect in a small town in the 14th century that the priest would
preach in the vernacular, and that the lessons would have been read in
Latin.

I expect that the present day lessons for Assumption Day would be appropriate.
Here are someone's choice in German:
http://www.cssr.at/prarchiv/a2002/amh_lesg.htm
    (from Revelations, I Corinthians, Luke)

And for a little color in your text, this excerpt:
"I would like to remind you of those ancient shouts of triumph which
echo throughout the Western Liturgies on August l5th: She IS taken up;
the Kingdom is hers; Assumpta est Maria in Caelum; exaltata est Sancta
Dei Genetrix; She in her Assumption put down all the heresies, all the
cowardliness, all the faithlessness; cunctas haereses sola interemisti
in universo mundo."

http://66.249.93.104/search?q=cache:sE2oyn0r7eEJ:www.allsaintsbrisbane.com/03_news/hunwickeandassumption.htm+%22assumption+day%22+lesson&hl=en&ct=clnk&cd=41
  (Only the cached site opened.)

As usual, I have to speculate:  will the hero be listening to a Latin
lesson and entertaining unholy thoughts, repenting them during the
homily; or will he be distracted from them by the obscure vernacular
texts, wondering what they have to do with the holy day, while he
wonders what will happen during the subsequent pre-harvest
blessing/celebration that takes place on August 15th?
  (I know, don't admonish me, the question is purely rhetorical.  :)

Cheers, Myoarin

Hi there, Myoarin--how nice to hear from you!  I was hoping you'd stop by.

Thank you very much for your helpful comments.  They bear out what I
expected would be the case, but I had not had much luck in verifying
it through research.

You're welcome to speculate, as ever.  Wouldn't it be interesting if
you went ahead and wrote the story you're imagining?  There are many
ways to connect the same dots.

Regards,
Tryx

Tryx, 
It's a pleasure, as always.  I have begone to wonder what percentage
of the population at that time understood some Latin, thinking that
anyone who could read probably did, since  - rather similar to Koran
schools -  Latin was the primary written language taught at the time. 
As the sites mention, there were vernacular books, that is, expensive
manuscripts, but I expect that anyone who knew their alphabet had
originally been exposed to Latin.
Just to confuse the issue ...

Next time I get in a discussion with local teachers who claim that one
can't really learn German until one has learned Latin (as they do), I
am going to argue that their attitude is a centuries' old
preconception and disregard for the vernacular.  Could be an
interesting new input to those discussions.  :-)

Anyway, write on.
Regards, Myoarin