Merge pull request #139 from piwik/translationupdates

automatic translation update
added new language: Vietnamese
fixed test
fixes #4250
 Fixes ValueError: invalid literal for int() with base 10: '-' http://forum.piwik.org/read.php?2,106854
Reverting part of https://github.com/piwik/piwik/pull/120/files
Adding tests with invalid request (see prev commit)
add more information to translation update pull requests