Add translations test

Add a new test suite to verify the correct usage of the t() functions.

For instance:
t2('%s test', '%d tests', 1);
is wrong, since gettext can't compile the .po file (for the above example
msgfmt exits with the error message
"format specifications in 'msgid_plural' and 'msgstr[0]' for argument 1
are not the same")
Remove useless line
Merge pull request #1796 from mlocati/add-translations-tests

Add translations tests
Fix output of localization test messages
Merge pull request #1798 from mlocati/fix-localization-test-error-messages

Fix output of localization test messages