[i18n] Languages with multiple plurals aren't properly supported

[Jackenmen]

As an example, I saw this code:

Red-DiscordBot/redbot/core/utils/chat_formatting.py

Lines 438 to 445 in f7e2617

	periods = [
	(_("year"), _("years"), 60 * 60 * 24 * 365),
	(_("month"), _("months"), 60 * 60 * 24 * 30),
	(_("day"), _("days"), 60 * 60 * 24),
	(_("hour"), _("hours"), 60 * 60),
	(_("minute"), _("minutes"), 60),
	(_("second"), _("seconds"), 1),
	]

For example, in Polish we have 2 plural forms beside the singular:
1 month -> 1 miesiąc (1)
2 months -> 2 miesiące (2-4, 22-24, 32-34, ...)
5 months -> 5 miesięcy (5-21, 25-31, 35-39, ...)

We might need some sort of i18n library eventually, but I think an equivalent of ngettext would be enough for now.

[Jackenmen]

Made an update, cause I totally screwed up the example 😄