OK, here is a beta that has the German version of the Flesh Reading Ease statistic. The code for getting the language is on jobs.py at line 167.
Code:
from calibre.utils.localization import get_lang
lang = iterator.opf.language
lang = get_lang() if not lang else lang
In this, lang is set to a three character language code such as "eng" or "deu". "get_lang()" gets the current interface language. This then passed to the statistic method which chooses the appropriate calculation. The pickle file is loaded and passed to the job, so choosing at the book level is a problem. There is a comment in the pickle load code about why this is done. Choosing based on the interface language, would be simple.
And thinking about the extra statistics, I have thought of a way that might work to handle this.
At the moment, there are five statistics: words, pages, Flesch Reading Ease, Flesch-Kincaid Grade and Gunning Fox Index. These are fixed and the options around them are where to store the results. My thought is to add the extra statistics, but make them selectable from a list. The word and page count would be kept as they are. For the others, have pairs of drop-down lists. The first of each pair lists the statistics. The second the column to store it in. With that, exactly which statistic used from the full set would be up to the user. I would probably limit this to three stats, but, with a little thought, it could be extended to as many as needed.
I haven't looked enough at the NLTK code to see how easy it would be to replace the version in the plugin with a more complete version. For the simpler statistics that use calculations similar to those already in place, adding them in this way should be practical.