Either the English name for the language or the two letter ISO639 code.
The following language names are understood (aliases follow the name):
* none - don't stem terms
* danish (da)
* dutch (nl)
* english (en) - Martin Porter's 2002 revision of his stemmer
* english_lovins (lovins) - Lovin's stemmer
* english_porter (porter) - Porter's stemmer as described in his 1980 paper
* finnish (fi)
* french (fr)
* german (de)
* italian (it)
* norwegian (no)
* portuguese (pt)
* russian (ru)
* spanish (es)
* swedish (sv)
an error will be thrown if the specified language is unknown.