[FEATURE] Makes quantity optional in translation methods
When using plural forms, the quantity to be used for finding
the correct plural form had to be given explicitly. This was
cumbersome when using a single numeric argument, as it meant
to pass the same value twice.
With this change the quantity is determined automatically if
a single numeric argument is given to translateById() or
translateByOriginalLabel() in Translator.
Also fixes a few I18n unit tests along the way.