Validate placeholders in locales against default locale

[xPaw]

For example:

english:

	"app_lowest_price": {
		"message": "SteamDB lowest recorded price is <b>$price$</b>",
		"placeholders": {
			"price": {
				"content": "$1",
				"example": "1,99€"
			}
		}
	},

another locale:

	"app_lowest_price": {
		"message": "Минимальная зафиксированная цена на SteamDB — <b>$price$</b>",
		"placeholders": {
			"price": {
				"content": "$2",
				"example": "1,99€"
			}
		}
	},

I would like this produce a warning because the placeholder in the non default locale uses "$2" instead of "$1".