Custom numbering style format

I want to use the letters numbering style for lists.
But I'm working in Welsh and the alphabet is different.
Is there any way of using a list of letters as the numbering style?

Or maybe through use of the dictionary or a script I could change the letters?

Maybe a script that converts numbering to text and replaces each character with the welsh alphabet.

The Welsh alphabet should be

a

b

c

ch

d

dd

e

f

ff

g

ng

h

i

l

ll

m

n

o

p

ph

r

rh

s

t

th

u

w

y

Any suggestions will be welcome

Diolch.