Bug 69551-add Vietnamese to timestripper_tests.py plus bug fixing

Added Vietnamese language to timestripper_tests.py.

Fixed also a bug in TimeStripper.last_match_and_replace() found
debugging Vietnamese.

Month was not replaced correctly.
Inserted proper logic to handle when months are expressed as digits or
in words.

Updated timestripper_tests.py to cover the logic in
last_match_and_replace(), using sites with the two formats.

Change-Id: I148663b7c694c499c194e993526ea8f928e4c174