Update plural_forms.cpp by tinygettext to reflect needs of translators

Some languages like sk in rP21907 have changed crucial line with information about how many plurals language uses and how to calculate correct one. However in plural_forms this line could not be matched with implemented functions so as result object with default settings have been returned what resulted in using only singular for translatePlural calls.

Affected languages (12):
sk, lt, uk, cs, pl, ro, mk, he, ga, cy, be, br

I am leaving old variants as they can be useful for another languages.

Differential Revision: https://code.wildfiregames.com/D2751
Comments by: wraitii