Clean up MozL10n API use in Cost Control #1057795

This is relating bug 1057795
Understood but from a Cost Control point of view, that function is responsible of localizing (I don't mind how is localizing, I only want to hide implementation) so, please, leave the name of the API unchanged and add a comment about the internals if you want to clarify.

In the other hand, I was reviewing the L20n API in the link you provide me and I would want to recommend you to change the names for `ctx.localize` and `ctx.ready`. None of them suggest they are events and the first forces the developer to implement localization. I think the first should be `onentitiesready` or, at least, `onlocalized` and the second, `onready`.