revisions to helper:

@LaurentGoderre, please look over the changes I made. If you're okay with them feel free to merge.  Thanks for doing this!

***

**pr**

* changes error messages to mention the name of the helper
* usages `export.i18n` for compatibility with any setup

Also:

* adds tests
* updates metadata to describe Laurent as author
* updates readme/docs
ha, I thought the same thing at first, it does seem to make more sense to namespace the languages in a `i18n` variable, since `language` is current on the same object as actual languages.