Use the plural form appropriate to the message, not the requested locale

[benjaminwinger]

If the message isn't available and the fallback is used, it would otherwise use the plural function for the requested locale, not the fallbackLocale.

This bug causes incorrect output, or even crashes if the requested locale and the fallback locale are different enough.

E.g.

i18n = require 'i18n'
i18n.load({en = {
    foo = {
        one = "A thing",
        other = "%{count} things",
    }
}})

print(i18n.translate("foo", {count=0})) -- "0 things"
i18n.setLocale("fr")
print(i18n.translate("foo", {count=0})) -- "A thing"
i18n.setLocale("pl")
print(i18n.translate("foo", {count=0})) --[[
luajit: i18n/interpolate.lua:7: attempt to index local 'string' (a nil value)
stack traceback:
        i18n/interpolate.lua:7: in function 'interpolateValue'
        i18n/interpolate.lua:52: in function 'localizedTranslate'
        i18n/init.lua:140: in function 'translate'
        test.lua:13: in main chunk
        [C]: at 0x560b0c93a238
]]