Do you mean something like:
Select "all" functions, then select the first function, then type the beginning of a function name and only the matching names are shown?

give it a try  :-)

there are two problems with this (although it is useful in some cases):
- it is not easily discoverable
- it does not search through the function descriptions/keywords/etc. You have to know the exact precise name of the function.

Also, functions are not localized. My native language is French; although I can understand English function names (and prefer to see them kept in the original language, strangely), I would like being able to see a localized function explanation in the pane below, because if that thing was localized, maybe it could be possible to include it in the search, making things even easier to find.

There is a different bug for localization of function names.  With a patch,
even.

Jeff, you wrote:
- it is not easily discoverable

Since virtually all gtk lists (in gnumeric and elsewhere) have this feature one would hope that users would pick up on it. But I agree that by itself it is not easily discoverable. 

I agree that a search in the descriptions would be nice!

This problem has been fixed in the development version. The fix will be available in the next major software release. Thank you for your bug report.

Note at this time we search through the function names and the short descriptions of the functions.