Add translations tests

Add a new test suite to verify the correct usage of the t() functions.

For instance:
```php
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`)