Presumably foreign month names can be parsed by using localized CSL terms lists--not sure about seasons.
Have you figured out how to deal with months/seasons? It would be great to sort June 2001 or Summer 2001 before September 2001 or Fall 2001, i.e. to parse the seasons as well. But if you display the parsed date (2001-09-01) first and have users roll over every time to make sure it's a month/season and not an exact date, it would be extremely confusing an inconvenient to the user.
Have you figured out how to deal with months/seasons?
... season sorting is tricky as items might originate from either the northern or the southern hemisphere
I'm concerned about how the parsing works inside Zotero-- if I enter "10 декабря 2009 г." in a date field, Zotero parses it as 2009-00-10. I hope that it does parse correctly for users in the "ru" locale, but there's no reason to expect that my data will all be in the locale that I prefer to use Firefox in.
I believe Zotero—due to the way locales are loaded—currently parses in the current locale and in English, but obviously that's not ideal. (And actually, it uses a combination of interface locale and bibliography locale, because day suffixes are/were only available in the interface locale. It also parses using only the month name abbreviations, which might not work as well for languages other than English?)
As a temporary, partial improvement, the date parsing indicator in the next major release will show the actual order of the component parts.