These results have been found in fr-FR and de-DE linguistic environment.
Write this short macro:

dim v as currency
dim a as string, b as string, c as string, d as string
v = 12345.6
a = Str(v)
b = CStr(v)
c = Format(v, "Currency")
d = Format(v, "# ##0.0000 \€")
MsgBox(a & chr(13) & b & chr(13) & c & chr(13) & d)

With fr-FR environment we get this (correct) result:
 12345.6
12345,6
12 345,60 €
12 345,6000 €

Change the first line of the macro: declare v as Currency instead of Double.
With fr-FR environment we now get this result:
 12345.6000
12345.6000
12345.6000
12345.6000

As you can see, CStr does not obey the decimal separator, Format does not obey the 
formatting strings.