Calendar: unset translatable=false

Remove translatable=false from preferences_week_start_day_default since it breaks the original functionality.

Original comment in strings.xml:

Settings option to set week start day. Can be translated to provide
different defaults for different countries/cultures.
7 = Saturday, 1 = Sunday, 2 = Monday

Also removed an untranslatable string to fix compiler warning

Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: Ied798a38f985797a93b2f5952f33b3f7020a5df5
Calendar: Update Greek translations

Signed-off-by: Michael Bestas <mikeioannina@gmail.com>

Change-Id: I1e4d2e3fca777f99554c6fe503cee5ec6333ab30