The function creates and locks a non-exported object apa_lang that is used by other
papaja-functions. apa_lang is a list containing localizations for document
elements such as abstract and title. The selected language is defined by the lang-parameter
in the documents yaml-header. Currently, English (default) and German ("german") are supported.
If the output document is MS Word (output: apa6_word) the function additionally creates
a title page and adds the abstract. You should, therefore, always call apa_prepare_doc at the
very beginning of the R Markdown document.