Detect incomplete pluralizations

[Gargron]

In languages like Arabic, a lot of keys are required for using pluralization: not simply "one" and "other", but "zero", "one", "two", "few" and "many", and there are many languages like that. Having missing keys there breaks the whole page. It would be nice to have another diagnostic command that would check locale files for presence of pluralized keys, and ensure they have all the pluralizations necessary for that locale.