Christian, 
can you help out here, ie. tell me what to change the strings to and
i'll make the patch?

I'm no coder and I havent looked at the present code, nor do I know if
this is at all doable in glade, but here's some pseudo attempt:

/* The following assumes the base values are in mm */
if (a4_size) {
  if (imperial_units) {
    width = mm_to_inches (a4_width);
    height = mm_to_inches (a4_height);
    /* Translators: The following is paper size, the values are width
and height in inches */
    size_msg = g_strdup_printf (_("%d\" x %d\""),
                                width,
                                height);
  } else {
    width = a4_width;
    height = a4_height;
    /* Translators: The following is paper size, the values are width
and height in mm */
    size_msg = g_strdup_printf (_("%d mm x %d mm"),
                                width,
                                height);
  }
  /* Translators: The paper size gets inserted here */
  a4_msg = g_strdup_printf (_("A_4 (%s)"),
                              size_msg);
}
/* Do the same for other paper sizes */

Also applies to

#: data/epiphany.schemas.in.h:39
msgid "Printing bottom margin (in inches)."

#: data/epiphany.schemas.in.h:41
msgid "Printing left margin (in inches)."

#: data/epiphany.schemas.in.h:43
msgid "Printing right margin (in inches)."

#: data/epiphany.schemas.in.h:45
msgid "Printing top margin (in inches)."

#: data/glade/print.glade.h:6
msgid "<b>Margins (Inches)</b>"

Now we use metric.

Gnomeprint-ifying epiphany will provide support for imperial units.

*** This bug has been marked as a duplicate of 120924 ***