TAPi18n._afterUILanguageChange should be called in startup()

[derwaldgeist]

I just noted in my app that the _afterUILanguageChange() hook was broken. Whereas it worked in this combination: tap:[email protected]' and [email protected], after the switch to tap:[email protected]and[email protected]the hook wasn't executed anymore. My hook was replaced by a generic function that just returnednull. After I put the hook setup in a Meteor.startup()block, however, it worked again. So I think it should be mentioned in the docs that the hook should be set on startup. Or, it should be ensured thattap:i18n` doesn't overwrite it if it already has been set.