>...it looks like the default language will be whatever was specified last time...
I don't think so - I haven't checked the lifecycle of these factories precisely, but I've used the same configuration pattern as in StopFilterFactory, for example.
If I use the following configurations in my schema.xml, each in its own <fieldtype>:
<filter class="solr.SnowballPorterFilterFactory" language="French" />
<filter class="solr.SnowballPorterFilterFactory" />
The log says
INFO: SnowballPorterFilterFactory: language=French
INFO: SnowballPorterFilterFactory: language=English
Which is what I expect.
Let me know if I missed something!