Steps to reproduce the problem:
1- Set the language for a document to Spanish
2- Start a numbered list with more than 15 lines
3- Edit the list properties to change from numbers (1,2,3) to characters (a, b,
c)

Result:
After item n comes item o

What should happen:
Because Spanish alphabet have 27 characters (ch and ll, being digraphs, must not be used on lists) with ñ between n and o, the list on
a Spanish document should use the Spanish alphabet.

Background:
The "Real academia española", the maximun authority on Spanish language, have a
service to ask linguistic questions here:
http://www.rae.es/RAE/Noticias.nsf/Home?ReadForm
(under "consultas lingüisticas"). I asked them the question is it is
proper to use the ñ or not on lists.
In a nutshell (their complete answer, with English translation, at the end of
this report) is:
The use of the ñ is perfectly valid. ch and ll should be avoided because they
are digraphs. If the document must be sent outside Spanish area the
international alphabet is then appropriate, but the user have the right to
select to use the ñ or not. 
In this case, having a new entry on the lists types will be useful

Original text:
En la creación de esquemas o en la ordenación de epígrafes con letras se
prescinde de la ch y la ll precisamente por ser dígrafos o conjuntos de dos
letras. En principio, no hay problema alguno en utilizar la ñ, dado que es una
letra del alfabeto español. Se puede optar por el criterio que prefiera siempre
que este se mantenga coherentemente en todo el trabajo. Ahora bien, si una
publicación va a tener difusión fuera del ámbito español sería preferible
utilizar solo las letras del alfabeto internacional, que no incluye la ñ. 

English translation
In the creation of schemes or arrangements of epigraphs with letters one does
without the ch and the ll precisely for being digraphs or sets of two letters.
In principle, there is no problem in using ñ, because it is a letter of the
Spanish alphabet. It is possible to choose any criterion you prefers providing
that this one is kept coherent in all the work. Now then, if a publication is
going to have diffusion outside of the Spanish area it would be preferable to
use only the letters of the international alphabet, which does not include the
ñ. 

BTW: on a side note, on the last years I had the opportunity to visit several
theatres on the north of Italy. On all those theatres the rows are "numbered" with letters... of the Italian alphabet: no trace of j, k, w, x or y.
So I think the problem is not only with Spanish.