Cannot Access Global Variable in Loop

You are right: it's trying to load LANG15 from "content" array rather than main array. I don't think build-in Parser library has a solution for you. But you can use another parser library (I use Mustache myself https://github.com/bobthecow/mustache.php), but it will require additional work of setting it up.

P.S. I can send you link to my drop-in version of the library that requires no changes in your code to use it (e.g. $this->parser->parse).