To fix the scope of the  variable defined in the lib/tcpdf/lang/*.php ...

...files the file needs to be included not using module_load_include so that the language array defined stays in accessible scope.
Not sure that `require_once()` is appropriate here... What would happen if `citation_exporter_create_pdf()` was called two (or more) times during the same request? Would `$l` be available to later invocations?