I have a div like this:

<div data-i18n="10">10</div>

This results in Uncaught TypeError: messageKey.indexOf is not a function in this line in i18n:

lBracket = messageKey.indexOf( '[' );

Since messageKey is a number, not a string.

I suggest the following fix, a few lines up:

messageKey = $this.data( 'i18n' ) + '',

Thereby forcing messageKey to be a string.