A long long time ago (during my Oracle 7 days), I once needed to update base table SYS.PROPS$. This action was needed to change the database NLS characterset of US7ASCII to a characterset that would support GERMAN. Based on a metalink note, updating the SYS.PROPS$ base table, was the only way to achieve this (or completely rebuild the environment) in those Oracle 7 days.
This procedure was tricky. If you updated it with the wrong, an unsupported character set or with a typo in the string, the database would be corrupted and could not be started up again (so be warned if you want to fiddle around with the method)
Since those days, I always lookup NLS settings via a quick select on that table. The last time I did this, was a long time ago and to my surprise, while looking up settings, I noticed that this table does contain more data then only NLS parameters these days…
Output of a full (demo) clean database Oracle 11g installation gives: